problem solving questions in react

30 ReactJS Coding Interview Questions for Beginner, Mid-Level and Expert Developers

ReactJS is one of the leading front-end development frameworks in the IT industry. It is part of the open-source development ecosystem and is widely used by many companies.

Today we will discuss how to design technical coding challenges for ReactJS developers. Some ReactJS concepts that should be evaluated include React Component Lifecycle, State management, Virtual DOM, Router, JSX and hooks.

We have divided the coding challenges into three sections based on the experience level of developers; beginner, mid-level and expert. Let’s start with coding challenges for beginner-level developers.

Interview Questions for Beginner ReactJS Developers

When preparing interview questions for beginner-level ReactJS developers, your questions should test basic javascript, props, state, JSX, life cycle methods, routing, and styling. Here are some sample questions in this regard:

What will be the output of the below code if the button is clicked:

Find the issue in the below code snippet after rendering the list of names.

Develop a currency converter application that allows users to input an amount in one currency and convert it to another. For the sake of this challenge, you can use a hard-coded exchange rate. Take advantage of React state and event handlers to manage the input and conversion calculations.

What will be the output when the user types in the input field:

What is missing in the below code snippet:

Create a stopwatch application through which users can start, pause and reset the timer. Use React state, event handlers and the setTimeout or setInterval functions to manage the timer’s state and actions.

Analyze the below code snippet and advise what will be shown on the screen when the App component is rendered with <App name=”Claire” />?

Find the issue with the form’s input field in the below code snippet:

Question 10

What issue exists in the below code regarding state variable:

Interview Questions for Mid-level ReactJS Developers

When designing interview questions for intermediate-level ReactJS developers, you should design challenges that test a deeper understanding of the framework and advanced features. Some of the areas that should be evaluated include advanced react concepts, Redux, Server-Side Rendering, API Integration, Advanced CSS and CI/CD. Find below some of the challenges that test these concepts:

Find the issue in the below code when using the index as a key for list items:

Analyze the below code and advise what will be the value of “Count” when the button is clicked:

Develop a messaging application that allows users to send and receive messages in real time. The application should display a list of conversations and allow the user to select a specific conversation to view its messages. The messages should be displayed in a chat interface with the most recent message at the top. Users should be able to send new messages and receive push notifications.

Build a weather application that retrieves real-time weather data from any weather API and displays it on the screen. The app should provide a search feature through which users can search for a location by city name or zip code and display the current temperature, humidity, wind speed and other relevant weather data.

What is the issue in the below code regarding useMemo hook:

What will be logged to the console after the button is clicked:

Will the below code throw any error when adding a new item to the list? If yes, what error will that be?

Build a simple drag-and-drop interface that allows users to reorder a list of items through drag-and-drop. Implement a visual cue to indicate the new position of the dragged item. Use React state, components and hooks to manage the list’s data and user interactions.

What is wrong with the below code:

Develop a paginated list view that shows items fetched from an API or a mock dataset. Implement a pagination system through which users can navigate between pages. Also, display the number of items per page. Use React state, components and hooks to manage the data and user interactions.

Interview Questions for Experts

When designing coding challenges for expert-level ReactJS developers, you should evaluate the advanced features of the language and the ability to develop applications in a distributed system. Some of the areas to evaluate include architecture and design patterns, performance optimization, testing, advanced react concepts, security and integration with other technologies. Here are some coding challenges which are suitable for expert ReactJS developers:

Find the issue in the below code:

What is the output of the Toolbar component in the below code snippet?

See the below code snippet and advise, will there be any issue making a REST API call in a component’s useEffect hook?

What will be the behavior of useEffect hook in the below code:

Develop a web application of your choice that makes frequent requests to an external REST API. To improve latency and reduce the load on the API, implement a client-side caching strategy that caches responses to API requests and serves them from the client-side cache when the same request is made again.

What is wrong with using async/await in a useEffect hook in reference to the below code snippet?

What will be the behavior of the useRef and useCallback hooks in the below code snippet?

Develop a file upload component to upload multiple files simultaneously. It should display progress indicators for each file and should display a success or error message after the upload is complete.

How can you optimize the handling of async data promises in the below code?

Develop a dashboard that shows real-time data from multiple sources, including REST APIs and websockets. The data should update in real time without needing to reload. The dashboard should display gracefully on mobile devices and should have a user-friendly UI/UX.

Today we went through 30 coding challenges for ReactJS developers. These coding challenges test the knowledge and skills of ReactJS engineers in various areas of ReactJS, ranging from basic ReacJS concepts to advanced concepts like architecture and design patterns.

By categorizing the coding challenges separately for beginner level to advanced level, we have made it easy for hiring managers to evaluate candidates based on their experience. By completing these challenges, developers can enhance their knowledge of ReactJS and gain valuable experience in solving relevant problems.

React Fundamentals

ReactJS Babel Introduction
ReactJS Virtual DOM
React JS ReactDOM
React Lists
React Forms
ReactJS Keys
ReactJS Refs
ReactJS Rendering Elements
React Conditional Rendering
Code Splitting in React
ReactJS | Components - Set 2
ReactJS Pure Components
ReactJS Functional Components
React Lifecycle
Differences between Functional Components and Class Components
ReactJS Container and Presentational Pattern in Components

React Props & States

ReactJS Methods as Props
ReactJS PropTypes
ReactJS Props - Set 1
ReactJS Props - Set 2
ReactJS Unidirectional Data Flow
ReactJS State
ReactJS State vs Props
Implementing State in React Components
React useState Hook
ReactJS useEffect Hook
Context in React
React Router
React JS Types of Routers
ReactJS Fragments
Create ToDo App using ReactJS
Create a Quiz App using ReactJS
Create a Coin Flipping App using ReactJS
How to create a Color-Box App using ReactJS?
Dice Rolling App using ReactJS
Guess the number with React

React Connection & Deployment

How to Deploy Your React Websites on GitHub?
How to Deploy React project on Firebase?
How to deploy React app to Heroku?
How to deploy React app to Surge ?
How to deploy simple frontend server-less (static) React applications on Netlify

React Exercises

React exercises, practice questions and solutions, react questions.

How to connect Django with Reactjs ?
7 React Best Practices Every Web Developer Should Follow
8 Ways to Style React Components
How to add Stateful component without constructor class in React?
How to display a PDF as an image in React app using URL?
React JS Toast Notification
What is the use of data-reactid attribute in HTML ?
How to zoom-in and zoom-out image using ReactJS?
How to avoid binding by using arrow functions in callbacks in ReactJS?
How to bind 'this' keyword to resolve classical error message 'state of undefined' in React?
How to get the height and width of an Image using ReactJS?
How to handle multiple input field in react form with a single function?
How to handle states of mutable data types?
How to change state continuously after a certain amount of time in React?
How to change the state of react component on click?

React Exercise: Explore interactive quizzes, track progress, and enhance coding skills with our engaging portal. Ideal for beginners and experienced developers, Level up your React proficiency at your own pace. Start coding now!

A step-by-step React practice guide for beginner to advance.

Benefits of React Exercises

Interactive Quizzes: Engage in hands-on React quizzes.
Progress Tracking: Monitor your learning journey.
Skill Enhancement: Sharpen coding skills effectively.
Flexible Learning: Practice at your own pace.
Immediate Feedback: Receive instant results and feedback.
Convenient Accessibility: Accessible online, anytime.
Real-world Application: Apply React concepts practically.
Comprehensive Learning: Cover a range of React topics.

How to Start Practice ?

Embark on your React learning journey with our online practice portal. Start by selecting quizzes tailored to your skill level. Engage in hands-on coding exercises, receive real-time feedback, and monitor your progress. With our user-friendly platform, mastering React becomes an enjoyable and personalized experience. Elevate your coding expertise today.

React Best Practice Guide

Dive into React excellence with our Best Practice Guide. Explore coding standards, optimization techniques, and industry-recommended approaches. Elevate your skills through insightful tips, real-world examples, and interactive challenges. Unlock the secrets of efficient and maintainable React coding, ensuring your projects stand out with clarity and performance.

Why Practice React Online ?

Elevate your React online test experience with categorized fill-in-the-blank questions. This structured format guides your learning, providing targeted challenges for hands-on improvement. Engage in focused practice as it reinforce coding concepts, receive instant feedback, and enhance your skills incrementally. Moreover, by combining theoretical understanding with practical application, you’ll enjoy a comprehensive and effective approach to mastering React and as a result optimizing your performance in real-world scenarios.

React Online Practice Rules

Be Honest: Complete challenges independently, refrain from plagiarism or seeking unauthorized help.
Manage Time: Adhere to assessment time limits, simulating real-world scenarios.
Code Quality : Prioritize clean, efficient, and well-documented code.
Follow Guidelines : Adhere to platform-specific instructions, including input/output formats and code submission procedures.
No Cheating: Avoid external resources during assessments, unless explicitly allowed.
Utilize Feedback: Learn from automated or mentor/community feedback.
Participate Actively: Engage in forums, discussions, and share insights with fellow learners.
Continuous Improvement: Use assessments to identify and improve areas of weakness for ongoing growth.

Features of Practice Portal :

Instant Mistake Correction: Receive immediate feedback on errors to facilitate quick learning.
Unlimited Tries: Allow learners to attempt questions multiple times for mastery.
Time Display for Each Category: Show elapsed time for each set of 5-6 fill-in-the-blank questions to manage time effectively.
Table View of Scenarios: Provide a comprehensive table view displaying all categories, their status (completed or pending), and corresponding scores.
Performance Analytics: Track progress with detailed analytics, highlighting strengths and areas for improvement.
Interactive Code Editor: Offer a responsive code editor for an immersive hands-on experience.
Hints and Solutions: Include hints for guidance and solutions to foster self-learning.
Community Integration: Facilitate collaboration through forums and discussions for peer learning.
Adaptive Difficulty: Adjust difficulty levels based on user performance for personalized challenges.
Gamification Elements: Incorporate scoring, achievements, or badges to make learning engaging.

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10 React JS Practice Exercises with solution

1. build search filter in react.

React code to build a simple search filter functionality to display a filtered list based on the search query entered by the user.

Please enable JavaScript

2. Simple counter exercise

Creating a simple counter using React which increments or decrements count dynamically on-screen as the user clicks on the button. This exercise requires knowledge of fundamental React concepts such as State, Component, etc.

We can complete the simple counter exercise with the following steps:

3. Display a list in React

React code to print each item from the list on the page using Array.map() function to display each item on the page.

The list of items is displayed using React JS through the following steps:

4. Build Accordion in React

Creating an accordion that toggles text content on click of the accordion header using React State and conditional rendering.

The following are the steps to create an accordion in React JS:

5. Image Slider using React JS

The following are the steps to create an image slider in React JS:

6. Create a Checklist in React

React code to display a checklist with multiple options that can select and the selected options are dynamically displayed on the screen. React State is used to keep track of checked options and onChange() Event handler is triggered to alter the state whenever an option is checked or unchecked.

The following are the steps to create a Checklist in React JS:

7. Simple Login form in React

React code for simple login form where the user login by entering their username and password. The form inputs are validated to check if correct information is entered and the error messages are the validation fails. The login form is hidden and the “Welcome, ${name}” message is shown when the user login is successful.

The following are the steps to create a simple login form using React JS:

8. Print data from REST API

React code to collect data from rest API using fetch() in JavaScript combined with useEffect() to load the content on page render.

The following are the steps to print data from REST API in React JS:

9. Multi-Page navigation using React Router

10. Context API in React Components

Context allows values to be passed from multiple levels of child components without using props. Thus context can be used as an alternative to Redux in some of the cases. Learn more about the context in React from reactjs.org/docs/context.html .

The following are the steps to use Context API using React JS:

21 Essential React.js Interview Questions *

Toptal sourced essential questions that the best react.js developers and engineers can answer. driven from our community, we encourage experts to submit questions and offer feedback..

Interview Questions

Explain the Virtual DOM, and a pragmatic overview of how React renders it to the DOM.

The Virtual DOM is an interesting concept; it’s a complex idea that boils down into a much simpler algorithm.

In React, if we create simple ES6 class and print it out, we have a function (as all functions can be used as a constructor in JavaScript):

The console.log(Comments) gives us something that looks like this (after compiled by Babel from ES6 to ES5):

When we write something to draw a React Component to the screen, we might have something like the following:

The JSX gets transpiled into ES5 by Babel as well:

We can see that <Comments /> is transpiled directly into React.createElement(Comments, null) . This is where we can see what a Virtual DOM object actually is : a plain JavaScript Object that represents the tag to be rendered onto the screen.

Let’s inspect the output of React.createElement() :

This gives us:

See how the type is a string? DOM.render({...}) gets this object above and looks at the type , and decides whether or not to reuse an existing <div> element on the DOM or create a new <div> and append it.

The Virtual DOM is not a simple Object – it is a recursive structure. For example, if we add two elements beneath the <div/> :

What we get is a nested Object-tree:

This is why, in a React Component’s code, we can access the child and ancestor elements via this.props.children . What React will do is walk down a very deep tree of nested Objects (depending on your UI complexity), each sitting in their parent element’s children .

One thing to note is that the type so far has just been a string. When a React Element is made from a custom Component (like Comments above), the type is a function:

You can play around with a web version of this code at Matthew Keas’ github .

Explain the standard JavaScript toolchain, transpilation (via Babel or other compilers), JSX, and these items’ significance in recent development. What sort of tools might you use in the build steps to optimize the compiled output React code?

The bleeding edge JavaScript toolchain can seem quite complex, and it’s very important to feel confident in the toolchain and to have a mental picture of how the pieces fit together.

There are a couple primary pillars in the JavaScript toolchain: Dependency Management, Linting, Style-checking, Transpilation, and Compilation, Minification, Source-Mapping.

Typically, we use build tools like Gulp, Watchify/Browserify, Broccoli, or Webpack to watch the filesystem for file events (like when you add or edit a file). After this occurs, the build tool is configured to carry out a group of sequential or parallel tasks .

This part is the most complex piece, and is the center of the development process.

The rest of the tools belong in that group of sequential or parallel tasks:

Style linting - typically a linter like JSCS is used to ensure the source code is following a certain structure and style
Dependency Management - for JavaScript projects, most people use other packages from npm; some plugins exist for build systems (e.g. Webpack) and compilers (e.g. Babel) that allow automatic installation of packages being import ed or require() ‘d
Transpilation - a specific sub-genre of compilation, transpilation involves compiling code from one source version to another, only to a similar runtime level (e.g. ES6 to ES5)
Compilation - specifically separate from transpiling ES6 and JSX to ES5, is the act of including assets, processing CSS files as JSON, or other mechanisms that can load and inject external assets and code into a file. In addition, there are all sorts of build steps that can analyze your code and even optimize it for you.
Minification and Compression - typically part of – but not exclusively controlled by – compilation, is the act of minifying and compressing a JS file into fewer and/or smaller files
Source-Mapping - another optional part of compilation is building source maps, which help identify the line in the original source code that corresponds with the line in the output code (i.e. where an error occurred)

For React, there are specific build tool plugins, such as the babel-react-optimize presets that involves compiling code into a format that optimizes React, such as automatically compiling any React.createElement() calls into a JavaScript Object that inlines right into the source code:

How compilers can help optimize React
How to bootstrap a modern toolchain with Create React App
The Next.js and GatsbyJS frameworks built on top of React

How would you create Higher Order Components (HOCs) in React?

Higher Order Components (HOCs) are the coined term for a custom Component that accepts dynamically provided children. For example, let’s make <LazyLoad /> Component that takes child image tags as children , waits until the <LazyLoad /> Component is scrolled into view, and then loads the images they point to in the background (before rendering them to the DOM).

An HOC accepts children via props:

Creating an HOC means handling this.props.children in the Component’s code:

interactive example can be found at https://goo.gl/ns0B6j

Noting a few things about this code:

We set up initial state ( this.state = {loaded: 0} ) in the constructor() . This will be set to 1 when the parent container is scrolled into view.
The render() returns the props.children as child elements when this occurs. Extract the src by using ES6 destructuring, where {props:{src}} creates a variable src with the appropriate value.
We used a single componentDidMount() lifecycle method. This is used because on mount, we’d like the component to check if the HOC is visible.
The largest function of our component, _scroll() , grabs the HOC Component’s DOM element with DOM.findDOMNode() and then gets the elements position. This position is compared to the height of the browser window, and if it is less than 100px from the bottom, then the scroll listener is removed and loaded is set to 1 .

This technique is called HOC (Higher Order Component) because we pass in elements as this.props.children when we nest those elements inside the container component:

All of these nested elements (which can be custom components) are nested under <HOC/> , thus HOC ’s code will be able to access them as this.props.children .

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What is the significance of keys in React?

Keys in React are used to identify unique VDOM Elements with their corresponding data driving the UI; having them helps React optimize rendering by recycling existing DOM elements. Let’s look at an example to portray this.

We have two <TwitterUser> Components being rendered to a page, drawn in decreasing order of followers:

Let’s say that B gets updated with 105 Twitter followers, so the app re-renders, and switches the ordering of A and B:

Without keys, React would primarily re-render both <TwitterUser> Elements in the DOM. It would re-use DOM elements, but React won’t re-order DOM Elements on the screen.

With keys, React would actually re-order the DOM elements, instead of rendering a lot of nested DOM changes. This can serve as a huge performance enhancement, especially if the DOM and VDOM/React Elements being used are costly to render.

Keys themselves should be a unique number or string; so if a React Component is the only child with its key, then React will repurpose the DOM Element represented by that key in future calls to render() .

Let’s demonstrate this with a simple list of todos rendered with React:

Interactive code sample available on Matthew Keas’ github .

The setInterval() occurring on mount reorders the items array in this.state every 20ms. Computationally, if React is reordering the items in state, then it would manipulate the DOM elements themselves instead of “dragging” them around between positions in the <ul> .

It is worth noting here that if you render a homogenous array of children – such as the <li> ’s above – React will actually console.warn() you of the potential issue, giving you a stack trace and line number to debug from. You won’t have to worry about React quietly breaking.

What is the significance of refs in React?

Similarly to keys, refs are added as an attribute to a React.createElement() call, such as <li ref="someName"/> . The ref serves a different purpose, it provides us quick and simple access to the DOM Element represented by a React Element.

Refs can be either a string or a function. Using a string will tell React to automatically store the DOM Element as this.refs[refValue] . For example:

this.refs.someThing inside componentDidUpdate() used to refer to a special identifier that we could use with React.findDOMNode(refObject) – which would provide us with the DOM node that exists on the DOM at this very specific instance in time. Now, React automatically attaches the DOM node to the ref, meaning that this.refs.someThing will directly point to a DOM Element instance.

Additionally, a ref can be a function that takes a single input. This is a more dynamic means for you assign and store the DOM nodes as variables in your code. For example:

[Legacy projects only, < circa 2016] In a general overview, how might React Router and its techniques differ from more traditional JavaScript routers like Backbone’s Router?

“Traditional” routers like the ever-popular Backbone.Router establish a predefined set of routes, in which each route defines a series of actions to take when a route is triggered. When combining Backbone.Router with React, you may have to mount and unmount React Components when the route changes:

The router exists externally of the React Components, and the VDOM has to mount and unmount potentially frequently, introducing a possible slew of problems. React Router focuses on not just “single-level” routing, but enables - nay, empowers - the creation of HOCs that can “decide for themselves” what to render within them.

This is where the advanced HOC implementations can really help simplify a seemingly complex notion. Let’s look at using a tiny router to assess some of the beauty of embedding application routers inside React HOCs. Here, we define a Component that wraps it’s own routing mechanism ( router() not provided here, see universal-utils ):

This Router Component opts for parsing the routes object passed into this.props instead of reading over an array of React Components passed as this.props.children . React Router opts for the latter technique. Need proof? Take a look at this example code provided by React Router’s docs :

A <Router /> Component has one or more <Route /> Components as items in this.props.children , and <Route /> s can have sub- <Route /> s. React Router’s code recursively walks down the tree of children until there are no more to process, allowing the developer to recursively declare routes in a structure that encapsulates sub-routes, instead of having to implement a Backbone-esque flat list of routes (i.e. "/" , "/about" , "/users" , "/users/:id" , etc).

Why do class methods need to be bound to a class instance, and how can you avoid the need for binding?

In JavaScript, the value of this changes depending on the current context. Within React class component methods, developers normally expect this to refer to the current instance of a component, so it is necessary to bind these methods to the instance. Normally this is done in the constructor—for example:

There are several common approaches used to avoid this binding:

1. Define Your Event Handler as an Inline Arrow Function

For example:

Using an arrow function like this works because arrow functions do not have their own this context. Instead, this will refer to the context in which the arrow function was defined—in this case, the current instance of SubmitButton .

2. Define Your Event Handler as an Arrow Function Assigned to a Class Field

Note: As of September 2019, class fields are a Stage 3 ECMAScript proposal and are not yet part of the published ECMAScript specification. However, they are available for use in both Google Chrome and Mozilla Firefox and are commonly used in React projects.

3. Use a Function Component with Hooks

Using the hooks functionality in React it is possible to use state without using this , which simplifies component implementation and unit testing.

Explain the positives and negatives of shallow rendering components in tests.

It is faster to shallow render a component than to fully render it. When a React project contains a large number of components, this performance difference can have a significant impact on the total time taken for unit tests to execute.
Shallow rendering prevents testing outside the boundaries of the component being tested—a best practice of unit testing.
Shallow rendering is less similar to real-world usage of a component as part of an application, so it may not catch certain problems. Take the example of a <House /> component that renders a <LivingRoom /> component. Within a real application, if the <LivingRoom /> component is broken and throws an error, then <House /> would fail to render. However, if the unit tests of <House /> only use shallow rendering, then this issue will not be identified unless <LivingRoom /> is also covered with unit tests.

If you wanted a component to perform an action only once when the component initially rendered—e.g., make a web analytics call—how would you achieve this with a class component? And how would you achieve it with a function component?

Using a Class Component

The componentDidMount() lifecycle hook can be used with class components:

Any actions defined within a componentDidMount() lifecycle hook are called only once when the component is first mounted.

Using a Function Component

The useEffect() hook can be used with function components:

The useEffect() hook is more flexible than the lifecycle methods used for class components. It receives two parameters:

The first parameter it takes is a callback function to be executed.
The optional second parameter it takes is an array containing any variables that are to be tracked.

The value passed as the second argument controls when the callback is executed:

If the second parameter is undefined , the callback is executed every time that the component is rendered.
If the second parameter contains an array of variables , then the callback will be executed as part of the first render cycle and will be executed again each time an item in the array is modified.
If the second parameter contains an empty array , the callback will be executed only once as part of the first render cycle. The example above shows how passing an empty array can result in similar behaviour to the componentDidMount() hook within a function component.

What are the most common approaches for styling a React application?

CSS Classes

React allows class names to be specified for a component, like class names are specified for a DOM element in HTML.

When developers first start using React after developing traditional web applications, they often use CSS classes for styling because they are already familiar with the approach.

Styling React elements using inline CSS allows styles to be completely scoped to an element using a well-understood, standard approach. However, there are certain styling features that are not available with inline styles. For example, the styling of :hover pseudo-classes.

Pre-processors Such as Sass, Stylus, and Less

Pre-processors are often used on React projects. This is because, like CSS, they are well understood by developers and are often already in use if React is being integrated into a legacy application.

CSS-in-JS Modules Such as Styled Components, Emotion, and Styled-jsx

CSS-in-JS modules are a popular option for styling React applications because they integrate closely with React components. For example, they allow styles to change based on React props at runtime. Also, by default, most of these systems scope all styles to the respective component being styled.

If you were working on a React application that was rendering a page very slowly, how would you go about investigating and fixing the issue?

If a performance issue such as slow rendering is seen within a React app, the first step is to use the Profiler tool provided within the React Developer Tools browser plugin, which is available for Google Chrome and Mozilla Firefox. The Profiler tool allows developers to find components that take a long time to render or are rendering more frequently than necessary.

One of the most common issues in React applications is when components re-render unnecessarily. There are two tools provided by React that are helpful in these situations:

React.memo() : This prevents unnecessary re-rendering of function components
PureComponent : This prevents unnecessary re-rendering of class components

Both of these tools rely on a shallow comparison of the props passed into the component—if the props have not changed, then the component will not re-render. While both tools are very useful, the shallow comparison brings with it an additional performance penalty, so both can have a negative performance impact if used incorrectly. By using the React Profiler, performance can be measured before and after using these tools to ensure that performance is actually improved by making a given change.

At a high level, what is the virtual DOM (VDOM) and how does React use it to render to the DOM?

The VDOM is a programming concept, providing a critical part of the React architecture. Rather than interacting directly with the DOM, changes are instead first rendered to the VDOM—a lightweight representation of the target state of the DOM.

Changes made to the VDOM are batched together to avoid unnecessary frequent changes to the DOM. Each time these batched changes are persisted to the DOM, React creates a diff between the current representation and the previous representation persisted to the DOM, then applies the diff to the DOM.

This abstraction layer for the DOM provides a simple interface for developers while allowing React to update the DOM in an efficient and performant manner.

What is prop drilling and how can you avoid it?

When building a React application, there is often the need for a deeply nested component to use data provided by another component that is much higher in the hierarchy.

Consider the following example components:

<EditUsersPage /> , which includes selectedUserAddress in its component state and renders a <User /> component
<User /> , which renders a <UserDetails /> component
<UserDetails /> , which renders a <UserAddress /> component
A <UserAddress /> component that requires the selectedUserAddress property stored in the <EditUsersPage /> state

The simplest approach is to simply pass a selectedUserAddress prop from each component to the next in the hierarchy from the source component to the deeply nested component. This is called prop drilling.

The primary disadvantage of prop drilling is that components that should not otherwise be aware of the data—in this case <User /> and <UserDetails /> —become unnecessarily complicated and are harder to maintain.

To avoid prop drilling, a common approach is to use React context. This allows a Provider component that supplies data to be defined, and allows nested components to consume context data via either a Consumer component or a useContext hook.

While context can be used directly for sharing global state, it is also possible to use context indirectly via a state management module, such as Redux.

What is the StrictMode component and why would you use it?

<StrictMode /> is a component included with React to provide additional visibility of potential issues in components. If the application is running in development mode, any issues are logged to the development console, but these warnings are not shown if the application is running in production mode.

Developers use <StrictMode /> to find problems such as deprecated lifecycle methods and legacy patterns, to ensure that all React components follow current best practices.

<StrictMode /> can be applied at any level of an application component hierarchy, which allows it to be adopted incrementally within a codebase.

What is the key architectural difference between a JavaScript library such as React and a JavaScript framework such as Angular? How would that impact the decision for a project to use one versus the other?

React enables developers to render a user interface. To create a full front-end application, developers need other pieces, such as state management tools like Redux.

Like React, Angular enables developers to render a user interface, but it is a “batteries included” framework that includes prescriptive, opinionated solutions to common requirements like state management.

While there are many other considerations when comparing React and Angular specifically, this key architectural difference means that:

Using a library such as React can give a project a greater ability to evolve parts of the system—again for example, state management—over time, when new solutions are created by the open source community.
Using a framework such as Angular can make it easier for developers to get started and can also simplify maintenance.

How can automated tooling be used to improve the accessibility of a React application?

There are two main categories of automated tools that can be used to identify accessibility issues:

Static Analysis Tools

Linting tools like ESLint can be used with plugins such as eslint-plugin-jsx-a11y to analyse React projects at a component level. Static analysis tools run very quickly, so they bring a good benefit at a low cost.

Browser Tools

Browser accessibility tools such as aXe and Google Lighthouse perform automated accessibility at the app level. This can discover more real-world issues, because a browser is used to simulate the way that a user interacts with a website. It is possible for many of these tools to run in a continuous integration environment such as Travis or Jenkins. Since these tools take longer to execute, many developers just run these tools within their local browser on an occasional basis, such as when reaching project milestones.

[Legacy projects only: React < 16.8] What are pure functional Components?

Traditional React Components as we have seen thus far are creating a class with class Example extends React.Component or React.createClass() . These create stateful components if we ever set the state (i.e. this.setState() , getInitialState() , or this.state = {} inside a constructor() ).

If we have no intention for a Component to need state, or to need lifecycle methods, we can actually write Components with a pure function, hence the term “pure functional Component”:

This function that returns a React Element can be used whereever we see fit:

You might notice that <Date/> also takes a prop – we can still pass information into the Component.

How might React handle or restrict Props to certain types, or require certain Props to exist?

You may recall a previous example that looked like the following (some parts of the code left out):

When rendering the <LazyLoad/> , we can pass in props (i.e. <LazyLoad top={0}/> ). Props are essentially inputs or values being passed down to one Component from the parent rendering context, and the code that passes the props to the element may not be compliant with your code. For example, top here seems to be just a number, but would I be able to verify that the prop is in-fact a number before my component is rendered? It’s certainly possible to write this code in each and every Component that uses props . However, React provides us a much simpler and shorter solution: Prop Types.

When using React’s non-minified development version (i.e. when building and testing in development), React will throw an error to alert you of any instances where a Prop is either missing or the wrong type. Above, top should always be a number .

We can make top a required prop by adding:

PropTypes can be used to test Props for any kind of value . Here’s a few quick type-checkers React has for JavaScript’s built-in types:

React.PropTypes.array ,
React.PropTypes.bool ,
React.PropTypes.func ,
React.PropTypes.number ,
React.PropTypes.object ,
React.PropTypes.string ,
React.PropTypes.symbol ,

We can also test that props are React and DOM types:

React.PropTypes.node ,
React.PropTypes.element ,

And we have the ability to test more complex types, such as “shapes”, “instances of”, or “collections of”:

React.PropTypes.instanceOf(Message) ,
React.PropTypes.oneOf(['News', 'Photos']) ,
React.PropTypes.oneOfType([ React.PropTypes.string, React.PropTypes.number, React.PropTypes.instanceOf(Message)])
React.PropTypes.arrayOf(React.PropTypes.number) ,
React.PropTypes.shape({ color: React.PropTypes.string, fontSize: React.PropTypes.number })

Use these PropTypes to produce errors and track down bugs. When used effectively, PropTypes will prevent your team from losing too much time in the debugging and documentation process, ensuring stricter standards and understanding of your growing library of Components.

[Legacy projects only: React < 15.5] Compare and contrast creating React Components in ES5 and ES2015 (also known as ES6). What are the advantages and disadvantages of using one or the other? Include notes about default props, initial state, PropTypes, and DisplayName.

Creating React Components the ES5 way involves using the React.createClass() method:

This Comments Component can now be rendered either inside another React Component or directly in the call to ReactDOM.render() :

ES5 Components have some particular qualities, which we’ll note:

Like the above example, to set the state to an initial value, create the getInitialState() function on the Component. What it returns will be the initial state for a Component when rendered.
Additionally, you can set the default props for the component to have a certain value with the getDefaultProps() method on the ES5 version.
The displayName is used in debugging and error reporting by React. If you use JSX, then the displayName is automatically filled out.
For some, it is common practice to denote a custom method added to a React Component by prefixing it with an underscore, hence _handleClick . _handleClick is passed as the onClick callback for a button in the code above. We can’t do this so easily in the ES6 API of React, because the ES5 version has autobinding , but the ES6 does not. Let’s take a look at what autobinding provides:

Auto-binding

Consider the following piece of code:

Invoking thing.speak() in the console will log "jen" , but pressing a key will log undefined because the context of the callback is the global object. The browser’s global object – window – becomes this inside the speak() function, so this.name becomes window.name , which is undefined .

React in ES5 automatically does autobinding, effectively doing the following:

Autobinding automatically binds our functions to the React Component instance so that passing the function by reference in the render() works seamlessly.

Creating React Components the ES6 way works a little differently, favoring the ES6 class ... extends ... syntax, and no autobinding feature:

Notice that in ES6, we have a constructor() that we use to set the initial state,
We can add default props and a display name as properties of the new class created, and
The render() method, which works as normal, but we’ve had to alter how we pass in the callback function. This current approach ( <button onClick={() => this._handleClick}>click me!</button> ) will create a new function each time the component is re-rendered; so if it becomes a performance bottleneck, you can always bind manually and store the callback:

Or with class fields syntax:

Many React utility libraries on npm provide a single function to bind all handlers in the constructor, just like React does.

[Legacy projects only: React < 15.5] Compare and contrast incorporating mixins and enforcing modularity in React Components. ( extend , createClass and mixins, HOCs) Why would you use these techniques, and what are the drawbacks of each?

Modularity is – in effect – something partially done with intention while coding, and partially done when refactoring afterwards.

Let’s first paint a scenario which we’ll model using each method above. Imagine we have three React Components: onScrollable , Loadable , and Loggable .

an onScrollable Component will listen to the window.onscroll event, and use a logging mechanism to record it
a Loadable Component will not render until one or more async requests have finished loading, and will use a logging mechanism to record when this occurs
a Loggable Component provides a logging mechanism, be it a console , a Winston Node.js logging setup on our own server, or some 3rd party logging service that records logs via JSON requests

First, let’s model this with React’s ES5 API and mixins .

Interactive code sample at Matthew Keas’ github .

Let’s note a few things about the above code:

There are three POJOs (Plain ol’ JS Objects) created, which hold lifecycle and/or custom methods.
When creating the Example Component, we add mixins: [Loggable, Loadable, onKeypress] , meaning that any functions from all three objects are included in the Example class.
Both onKeypress and Loadable add a componentDidMount() , but this doesn’t mean the latter cancels out the prior. In fact, all componentDidMount() functions from each mixin will be invoked when the event occurs. The same is true for all lifecycle methods added to mixins. This way, both the onKeypress and Loadable mixins will work simultaneously!

Mixins are possible, but not built-in to React’s ES6 API. However, the ES6 API makes it easier to create a custom Component that extends another custom Component.

So our Components’ prototype chains would look like the following:

This would result from Components written as such:

Creating anonymous classes would help here, because then Loggable would not have to extend Loadable and onKeypress.

With a mixin() function, this could look more like:

Let’s try to write mixin() by building a chain of anonymous classes that extend Loggable , Loadable , and onKeypress :

There’s a caveat, though – if Loadable extends onKeypress and both implement componentDidMount() , Loadable ’s version will be lower on the prototype chain, which means the function from onKeypress will never be invoked.

The takeaway here is that the mixin pattern isn’t easily implemented by relying only on the ES6 extends approach. Let’s try to implement mixin() again, but build a more robust function:

This new mixin() implementation maps over each class, and cascades function calls from a parent class’s componentDidMount() alongside the child’s componentDidMount() .

There are similar implementations of mixin() available on npm, using packages like react-mixin and es6-react-mixins .

We use mixin() from above like so:

interactive code sample available at https://goo.gl/VnQ21R

Recently, React provided support for – and documented its preference of – React Components declared with ES6 classes. ES6 classes allow us to create component heirarchies with less code, however this makes it more difficult to create a single Component that inherits properties from several mixins, instead forcing us to create prototype chains.

[Legacy projects only: React < 16] Compare and contrast the various React Component lifecycle methods. How might understanding these help build certain interfaces/features?

There are several React lifecycle methods that help us manage the asynchronous and non-determinate nature of a Component during it’s lifetime in an app – we need provided methods to help us handle when a component is created, rendered, updates, or removed from the DOM.

Let’s first classify and define the life-cycle methods:

The “Will’s” - invoked right before the event represented occurs.

componentWillMount() - Invoked once, both on the client and server, immediately before the initial rendering occurs. If you call setState within this method, render() will see the updated state and will be executed only once despite the state change.
componentWillReceiveProps(object nextProps) - Invoked when a component is receiving new props. This method is not called for the initial render. Calling this.setState() within this function will not trigger an additional render. One common mistake is for code executed during this lifecycle method to assume that props have changed.
componentWillUnmount() - Invoked immediately before a component is unmounted from the DOM. Perform any necessary cleanup in this method, such as invalidating timers or cleaning up any DOM elements that were created in componentDidMount.
componentWillUpdate(object nextProps, object nextState) - Invoked immediately before rendering when new props or state are being received. This method is not called for the initial render.

The “Did’s”

componentDidMount() - Invoked once, only on the client (not on the server), immediately after the initial rendering occurs. At this point in the lifecycle, you can access any refs to your children (e.g., to access the underlying DOM representation). The componentDidMount() method of child components is invoked before that of the parent component.
componentDidUpdate(object prevProps, object prevState) - Invoked immediately after the component’s updates are flushed to the DOM. This method is not called for the initial render. Use this as an opportunity to operate on the DOM when the component has been updated.

The “Should’s”

shouldComponentUpdate(object nextState, object nextProps) - Invoked before rendering when new props or state are being received. This method is not called for the initial render or when forceUpdate() is used. Use this as an opportunity to return false when you’re certain that the transition to the new props and state will not require a component update.

Having a strong understanding of how these fit together – and how setState() or forceUpdate() affect the lifecycle – will help the conscious React developer build robust UIs.

There is more to interviewing than tricky technical questions, so these are intended merely as a guide. Not every “A” candidate worth hiring will be able to answer them all, nor does answering them all guarantee an “A” candidate. At the end of the day, hiring remains an art, a science — and a lot of work .

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Real-World React Coding Challenges For Your Interview Preparation

Navigating coding challenges is a crucial part of the interview process, particularly for aspiring React developers lacking professional experience.

Unfortunately, these coding assignments can be daunting for entry-level devs with limited job opportunities, making a failed test disheartening and even humiliating.

To help you prepare for code challenges, this article offers:

4 practical React coding challenges covering skills like CSS, simple state management, API data handling, and debugging existing codebases.
Insight into employers' objectives for coding challenges.
Guidance on tackling coding challenges during interviews.

React Code Challenges

Before diving in, it's recommended to read the entire article. However, let's kick off with the most exciting part: coding challenges you might encounter during a job interview or in your daily work as a developer.

These challenges stem from the codebase and designs of the React Job Simulator ( source code on GitHub ). They vary in difficulty and assess a range of skills.

Now, let's begin with an easy task.

1. Style A Header Based on Designs

This simple UI challenge involves creating a header component for the app's landing page using Figma designs.

Skills tested : CSS

Difficulty: Easy

Implement the header according to designs.
Navigation links should target / , /products , /documentation , and /pricing .
Adjust the existing "Open Dashboard" link to match the designs.
Edit pages/index.jsx in the repository.
styled-components is used for CSS; you may use this or alternatives like CSS modules or inline styles.
The project is in TypeScript, but the challenge file is JSX. Feel free to change to .tsx if preferred.
For help with designs or achieving pixel-perfect results, provide your email below for additional resources.

The solution for this task is not the cleanest, but it's sufficient for this challenge without separating the header into a new component file, using TypeScript, or writing tests.

You can get a more elaborate refactoring of above solution by providing your email below.

a production-grade code base
realistic tasks & workflows
high-end tooling setup
professional designs.

2. Toggle A Modal

This challenge is more dynamic, requiring you to create a modal that opens when users click the contact button.

Skills tested : CSS, simple state management, React APIs

Difficulty: Easy - Medium

Implement the modal UI.
The "Cancel" button should close the modal, while "Open Email App" should open users' email clients.
The modal should open upon clicking the contact button.
Don't use a modal library.
It's fine to add code to the file mentioned above.
There are two modal implementation methods: a simple one and a more advanced one. For both solutions, provide your email below.

For a tougher challenge:

Integrate your code with the existing codebase.
Add a Cypress test for this challenge to the existing test suite.
Add a Storybook story for the modal component.

These tasks are beyond the scope of this post. For more, check out React Job Simulator .

3. Render Elements Based on API Data from a CMS

This more advanced challenge involves connecting the website to a Content Management System (CMS) like Contentful or Strapi. Marketers can use a CMS to easily create content without developer assistance.

In this challenge, you'll create a content element controlled by API data.

Skills tested : Working with API data, CSS

Difficulty: Medium

Fetch data from the endpoint [https://prolog-api.profy.dev/content-page/home](https://prolog-api.profy.dev/content-page/home) , where home is a "slug" identifying the page.
Render the hero section (highlighted in the design) using that data.
Ensure the hero section matches the design.
Fetch data using any method.
Add code to pages/index.jsx .
API Swagger documentation is available here . (Provide your email below for more information)

These tasks are beyond this post's scope. For more, check out React Job Simulator .

4. Find The Bug

This challenge focuses on debugging, a crucial skill for developers. A bug from the React Job Simulator is provided for this exercise.

On the application's issue page , either the "Events" or "Users" column displays incorrect data.

Skills tested : Debugging an existing codebase

Identify the cause of the bug.
Fix the bug.
Adopt a structured approach rather than diving headfirst into the codebase. Use your browser's debugging tools instead of randomly probing the code.

The (Lesser Known) Purpose of Coding Challenges in the Interview Process

Practicing coding challenges is essential, but understanding what interviewers are looking for is equally important.

Employers want proof of a candidate's skills to minimize risks associated with hiring and training. Coding challenges are one way to assess these skills, testing both technical and soft skills.

While technical skills are crucial, don't overlook the importance of soft skills. Coding challenges can demonstrate your communication and problem-solving abilities, allowing interviewers to follow your thought process and observe your attitude towards critique.

This is particularly true for live coding challenges or when discussing your take-home assignment in a follow-up interview. Be prepared for questions on both technical and soft skills.

6 Tips for Coding Challenges in Job Interviews

Coding challenges can be stressful, especially live assignments with the interviewer.

Here are some tips on how to approach them:

Ask for Clarification

One common mistake, particularly among juniors, is diving straight into the task without fully understanding the requirements. Instead, take time to re-read the assignment and ask for clarification if something is unclear.

Asking clarifying questions is not a weakness. It demonstrates a clear mind and a structured approach, which employers value. On the job, building a solution without fully understanding the problem can lead to wasted time and effort.

Talk While You're Coding (For Live Challenges)

Live coding challenges can be intimidating but offer a unique opportunity to showcase your thought process. Talking out loud while writing code allows interviewers to see your problem-solving and communication skills in action.

However, thinking and talking simultaneously isn't easy for everyone. To improve, practice techniques like rubber duck debugging , where you explain your plans as you work on a task. The coding challenges mentioned earlier are excellent opportunities for practicing this skill.

Clean Code Format

Many inexperienced developers overlook the importance of clean code formatting. Inconsistencies in indentation, semicolons, or variable naming can be easily noticed by senior developers or interviewers.

To avoid these issues, use a code formatter like Prettier for your (take-home) coding assignments. This helps maintain consistency and demonstrates your attention to detail.

Use TypeScript and Write Tests If You Can

Experienced developers value the reliability provided by TypeScript and automated tests. If you're comfortable with it, use types in your code and add tests. The choice of tool doesn't matter – you can use Jest, React Testing Library, or Cypress. Even one or two tests can be enough to demonstrate your professionalism and commitment to code quality.

Add a README with Clear Instructions

Your interviewers may not run your code on their local machine, but if they do, you want to avoid confusion.

Include clear instructions on how to install and run your code, and double-check that it's working. If you have extra time, consider adding additional information to the README, such as:

Your approach to solving the assignment and the reasons behind it.
Any assumptions you made during the process.
Improvements you would make if given more time.

Be Prepared for Questions

Adding the information mentioned above to your README not only helps your interviewer understand your thought process, but it also prepares you for the next step: discussing your code.

If you have the opportunity (e.g., with a take-home test), let the code rest for a bit after submission. If you're invited to a follow-up interview, review your code closely:

What seems strange or confusing in hindsight?
Are there parts that are particularly difficult to understand?
How could you improve or refactor your code?
Is there anything you would do differently now?

Reevaluating your code can help you prepare for the next stage of the interview process, as it's likely you'll face questions about your code assignment.

55+ Top React Interview Questions and Answers to Prepare For

how to answer React Interview Questions for React.js

We’ve got the React interview questions and answers to practice and prepare for your big technical interview and land the job you want.

React has consistently been ranked among the top web frameworks and is widely used to build various applications. In fact, according to Stack Overflow’s 2020 developer survey , it’s ranked at number 2 out of all web frameworks, just behind jQuery!

React is a JavaScript library with a declarative and component-based approach for building user interfaces. React has exploded in popularity because its simple and declarative API produces highly-performant applications — and that momentum only continues to grow.

In this guide, we break important React interview questions to ask down into three groups:

Basic React Interview Questions

Intermediate React Interview Questions

Advanced React Interview Questions

You can expect to learn about what questions you should ask, what answers to expect, and what to focus on evaluating for each answer. Additionally, we’ll give you the reasoning behind why you should ask these questions and what to look for: why should you ask these questions? What do you want to know by asking your candidates these questions? How can you draw out better answers from your candidates?

Let’s get started!

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The following questions should test the candidate’s basic knowledge of React and some of its core features.

1. How to create components in React?

Components form the fundamental building blocks of a React application. The candidate should be able to explain the two approaches to creating a component in React along with their differences and applications.

React provides two ways to create a component – function components and class components.

Function Components is the simplest way to create a component. It uses pure JavaScript functions that accept props and returns a React element.
Class Components , on the other hand, uses ES6 class to define a component. The same function component can be rewritten as the following class component:

2. What are props in React?

Props are one of the most basic ways of passing data between components in React. The candidate should be able to explain what props are and how they’re used to communicate between different components. Don’t forget to provide a couple of examples to demonstrate its usage in a real application.

When building an application, you often end up building a component tree with a hierarchy of components. Oftentimes, you will need to pass data between the components as you go down the tree. These are where props come into play.

Props are the component’s inputs. They can be in the form of a single value or an object containing a set of values that are passed from a parent component to a child component.

The snippet below shows how a prop is passed from the parent:

The props can then be accessed via the child component’s props property:

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3. What are states in React?

A likely follow-up to the previous question, still in the realm of managing data inside a component. Understanding the concept of state in React is vital to being able to use components effectively. Make sure your candidate is able to explain what states are and how they are used in a React component.

State is an object that contains information local to the component that may change over the lifetime of the component.

The snippet below shows a state object containing the property message which is then rendered in the h1 tag.

4. What is context in React?

Context is a data-sharing mechanism in React. You might ask this question to gauge the candidate’s knowledge of how components can pass and share data. The candidate should be able to explain what context is and their ideal use cases.

Context is an alternate way to pass data through the component tree without passing props manually at every level. Context is designed to share data that is considered “global” within a tree of components.

For example, if you have a component tree with the following structure:

Passing data from component A to component D using props would require the data to be passed from A to B to C and finally to D. Context provides a way to share values like these between components to avoid the above scenario.

A common use case of context is application theming, language, and user information which are accessed from various parts of the application. The example below shows how user information is set and accessed by different components in the application.

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5. How do you conditionally render components?

Conditionally rendering a component is a common requirement in most applications. For example, a component might need to be rendered only if a certain set of data is available in your application’s state. Your candidate should explain the different approaches available and how to use them.

JSX doesn’t render false or undefined . This behavior lets us use conditional short-circuiting to render parts of your component using the && notation.

For example, a component that only displays the p tag when the name property exists can be written as:

A ternary operator can also be used to create an if-else condition. Using the same example above, we could add a fallback string that displays “Name not available” when name is undefined.

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6. How to bind methods or event handlers in JSX callbacks?

Handling events are a common requirement when building applications. These events could be DOM events, such as clicks or custom events that a component emits. A candidate should be able to explain how events work along with the various ways to listen to the emitted events.

React provides three approaches to listen to emitted events:

Callback with arrow functions – Bind the event to an arrow function that calls the desired function.
Bind in constructor – Event handlers defined as class methods in React aren’t bound by default. We could manually bind them in the constructor to allow them to be called from the template.
Public class fields syntax – We can avoid the manual binding of the method defining the method using the arrow function. Arrow functions don’t have their own this keyword causing the this keyword to be bound lexically.

7. What is a SyntheticEvent in React?

SyntheticEvent provides a convenient abstraction over browser events. This abstraction streamlines the interaction with browser events from our React application. You might ask this question to gain insight into the candidate’s level of understanding of events and how they work in React.

SyntheticEvent is a cross-browser wrapper around the browser’s native event. It has the same API as its browser’s native counterpart, including methods like stopPropagation() and preventDefault() . However, unlike the browser’s native events, SyntheticEvent works identically across all browsers.

Read More : 8 Questions to Ask Recruiters Before Committing to the Dev Hiring Process

8. How to conditionally apply class attributes?

Similar to hiding and displaying an element, sometimes, we also need to conditionally apply styling to our element. Typically, styles are applied via classes instead of directly setting it on the element. You might ask this question to understand the candidate’s familiarity with using conditionals in React and how they can be applied to class attributes.

React lets you include conditionals such as && and ternary operators to conditionally display parts of your component. The same concept can be applied to classNames.

The snippet below uses a ternary operator to add the disabled class when the disabled prop is true and the default class if it’s false.

9. What are refs in React?

Sometimes accessing the DOM directly is required. Understanding the recommended approach to accessing the DOM could save you from unexpected behaviors in your application. The candidate should be able to explain the purpose of refs, how refs are used, along with when they should be used.

Refs provide a way to access DOM nodes or React elements created in the render method.

Refs act as an escape hatch to the regular React dataflow of passing props down a component tree. They are used to update the child component (including both React components and DOM elements) without passing in new props.

The following snippet shows how refs are used to set the focus on the input element when a button is clicked.

A few use cases for refs include:

managing focus and text selection
triggering imperative animations
integrating with third-party DOM libraries

10. What are the four stages a React component goes through?

Or, asked another way: What happens during the lifecycle of a React component?

One of the most valuable parts of React is its component lifecycle — so understanding exactly how components function over time is instrumental in building a maintainable application.

Each React component has its own lifecycle. Each stage of the lifecycle invokes a series of methods allowing us to perform specific tasks at a specific stage of the component’s lifecycle. Understanding the different stages within a component’s lifecycle enables us to use them effectively. The candidate should be able to iterate what the four stages are and what happens in each stage.

A React Component goes through the following four stages:

Initialization – the component is constructed with the given props and default state
Mounting – rendering the JSX returned by the render method
Updating – when the state of a component is updated and the application is repainted
Unmounting – the final stage of the component lifecycle where the component is removed from the DOM

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11. What is React? How is it different from other JS frameworks?

Although this sounds like a relatively simple question, it’s asking the candidate to state an informed opinion about React and any competing alternatives. In short, this question is designed to test a candidate’s knowledge about the JavaScript ecosystem at large while also pressing for specifics on what makes React unique.

Let’s look at each part of the answer separately.

What is React?

React is an open-source JavaScript library created by Facebook for building complex, interactive UIs in web and mobile applications.

The key point in this answer is that React’s core purpose is to build UI components; it is often referred to as just the “V” (View) in an “MVC” architecture. Therefore it has no opinions on the other pieces of your technology stack and can be seamlessly integrated into any application.

How is React different?

The answer to this question will likely vary depending on the candidate’s personal experiences. The important thing is to listen for real-life examples provided and opinions on whether or not the candidate prefers React and why.

Because React is a small library focused on building UI components, it is necessarily different from many other JavaScript frameworks.

For example, AngularJS (1.x) approaches building an application by extending HTML markup and injecting various constructs (e.g. Directives, Controllers, Services) at runtime. As a result, AngularJS is very opinionated about the greater architecture of your application — these abstractions are certainly useful in some cases, but in many situations, they come at the cost of flexibility.

By contrast, React focuses exclusively on the creation of components, and has few (if any) opinions about an application’s architecture. This allows a developer incredible flexibility in choosing the architecture they deem “best” — though it also places the responsibility of choosing (or building) those parts on the developer.

“ I recently migrated an application originally written in Angular to React, and one of the things I loved most was… “

By comparing and contrasting React with another library, not only can candidates demonstrate a deep understanding of React to the interviewier, but also position themself as potentially strong candidate. Navigate React.js interview questions with confidence by gaining insights from industry experts and utilizing top-notch React.js development services .

Be prepared to ask some follow-up questions as well, such as:

Under what circumstances would you choose React over another technology? For example, React vs Angular or React vs Vue .
If React only focuses on a small part of building UI components, can you explain some pitfalls one might encounter when developing a large application?
If you were rewriting an Angular application in React, how much code could you expect to re-use?

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12. What can you tell me about JSX?

When Facebook first released React to the world, they also introduced a new dialect of JavaScript called JSX that embeds raw HTML templates inside JavaScript code. JSX code by itself cannot be read by the browser; it must be transpiled into traditional JavaScript using tools like Babel and webpack.

While many developers understandably have initial knee-jerk reactions against it, JSX (in tandem with ES2015) has become the defacto method of defining React components.

Asking questions about JSX tests whether or not the candidate can state an informed opinion towards JSX and defend it based on personal experience. Let’s cover some of the basic talking points.

Key Talking Points

Developers do not have to use JSX (and ES2015) to write an application in React. Having said that, many React developers prefer to use JSX as its syntax is far more declarative and reduces overall code complexity. Facebook certainly encourages it in all of their documentation!

Adopting JSX allows the developer to simultaneously adopt ES2015 — giving immediate access to some wonderful syntactic sugar.

ES2015 introduced a variety of new features to JavaScript that makes writing large applications far easier than ever before: classes, block scoping via let, and the new spread operator are just a small portion of the additions.

But while ES2015 is becoming more and more widespread, it still is far from widely supported by the major browsers — so tools like Babel or webpack are needed to convert everything into legacy ES5 code.

Candidates that have built a React application using JSX and ES2015 can speak about some specific pros or cons encountered, such as:

Although it took me some time to get used to the JSX and ES2015 syntax, I discovered how much I really enjoyed using it. Specifically, I’m a big fan of…

On the other hand, I could do without the hassle of configuring webpack and Babel. Our team ran into issues with…

The React docs on JSX Gotchas may be good to know/review.

13. What are stateless components?

If React components are essentially state machines that generate UI markup, then what are stateless components?

Stateless components (a flavor of “reusable” components) are nothing more than pure functions that render DOM-based solely on the properties provided to them.

This component has no need for any internal state — let alone a constructor or lifecycle handlers. The component’s output is purely a function of the properties provided to it.

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Intermediate React Interview Questions and Answers

The following set of questions should test a candidate’s mid-level knowledge of React and some of its core features.

1. How to create elements in a loop in React?

Applications usually involve displaying a dynamic set of components based on an array or object. The candidate should be able to explain what a v-for directive is along with their applications.

React lets us use JavaScript’s map function to loop through an array or object – rendering the returned template for each entry.

The following code shows how the items array is used to generate a set of ItemComponent :

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2. How do you update the state object in React?

Keeping your views and your logic in sync is an important aspect of every application. This is equivalent to updating the state object and making sure the component is updated accordingly. The candidate should be able to explain the recommended approach when working with the state object and how to avoid the component from getting out of sync with the state.

React provides utility functions to manage your component’s local state. The setState function is the recommended approach to update the state object as React does the heavy lifting for you.

The setState function merges the new object with the current state. The updated state then triggers a component re-render automatically, ensuring that your views are always up-to-date with the latest state changes in your component.

The following example shows how the user object is updated using the setState function:

3. What are Higher-order components ?

Higher-order components (HOC) is a pattern derived from React’s compositional nature. You might ask this question to better understand the candidate’s familiarity with React’s component system. The candidate should be able to explain what HOCs are conceptually and what problem it solves.

HOC is a function that accepts a component and returns a new component. These components are also referred to as pure components as they accept dynamic child component but doesn’t modify or copy any of the child components.

Use cases of HOCs include:

Code reuse and logic abstraction
State abstraction and manipulation
Props manipulation
Render hijacking

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4. What is the purpose of the useMemo hook?

Optimization of processes is a necessity as applications grow and begins to perform heavier tasks. Memoization is an optimization technique that helps speed up expensive function calls using cached results. Understanding optimization techniques is important to keep your code fast and efficient. The candidate should be able to explain what memoization is and also how to use React’s useMemo hook to optimize your application.

Memoization is an optimization technique that speeds up your code by storing the results of expensive function calls and reusing the stored result when the same input occurs again.

Functional components use the useMemo hook to memoize expensive functions. As a result, these functions are only called when the input changes instead of on every render.

The following code shows a memoized function using the useMemo hook. useMemo will only recompute the memoized value when one of the dependencies ( a or b ) has changed.

5. What are Render Props ?

Understanding different techniques to share code between components helps make an informed decision on the best approach . The candidate should have a basic understanding of what render props are, along with their ideal use cases.

Render prop is a technique to share code between components by passing a function (the function should return an element) as the component’s props. The component with the render prop can then use the element from the passed-in function in its render function to compose a new component.

The code below shows how a function is passed in as a prop to the DataProvider component:

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6. Are you familiar with Flux?

Flux is an architectural pattern that enforces unidirectional data flow — its core purpose is to control derived data so that multiple components can interact with that data without risking pollution.

The Flux pattern is generic; it’s not specific to React applications, nor is it required to build a React app. However, Flux is commonly used by React developers because React components are declarative — the rendered UI (View) is simply a function of state (Store data).

Flux is relatively simple in concept, but in a technical interview, it’s important that the developer demonstrates a deep understanding of its implementation. Let’s cover of the important few discussion points.

Description of Flux

In the Flux pattern, the Store is the central authority for all data; any mutations to the data must occur within the store. Changes to the Store data are subsequently broadcast to subscribing Views via events. Views then update themselves based on the new state of received data.

To request changes to any Store data, Actions may be fired. These Actions are controlled by a central Dispatcher; Actions may not occur simultaneously, ensuring that a Store only mutates data once per Action.

The strict unidirectional flow of this Flux pattern enforces data stability, reducing data-related runtime errors throughout an application.

Flux vs MVC

Traditional MVC patterns have worked well for separating the concerns of data (Model), UI (View) and logic (Controller) — but many web developers have discovered limitations with that approach as applications grow in size. Specifically, MVC architectures frequently encounter two main problems:

Poorly defined data flow: The cascading updates which occur across views often lead to a tangled web of events which is difficult to debug.
Lack of data integrity: Model data can be mutated from anywhere, yielding unpredictable results across the UI.

With the Flux pattern complex UIs no longer suffer from cascading updates; any given React component will be able to reconstruct its state based on the data provided by the store. The flux pattern also enforces data integrity by restricting direct access to the shared data.

During a technical interview, one should discuss the differences between the Flux and MVC design patterns within the context of a specific example:

For example, imagine we have a “master/detail” UI in which the user can select a record from a list (master view) and edit it using an auto-populated form (detail view).

With an MVC architecture, the data contained within the Model is shared between both the master and detail Views. Each of these views might have its own Controller delegating updates between the Model and the View. At any point the data contained within the Model might be updated — and it’s difficult to know where exactly that change occurred. Did it happen in one of the Views sharing that Model, or in one of the Controllers? Because the Model’s data can be mutated by any actor in the application, the risk of data pollution in complex UIs is greater than we’d like.

With a Flux architecture, the Store data is similarly shared between multiple Views. However this data can’t be directly mutated — all of the requests to update the data must pass through the Action > Dispatcher chain first, eliminating the risk of random data pollution. When updates are made to the data, it’s now much easier to locate the code requesting those changes.

Difference with AngularJS (1.x)

UI components in AngularJS typically rely on some internal $scope to store their data. This data can be directly mutated from within the UI component or anything given access to $scope — a risky situation for any part of the component or greater application which relies on that data.

By contrast, the Flux pattern encourages the use of immutable data. Because the store is the central authority on all data, any mutations to that data must occur within the store. The risk of data pollution is greatly reduced.

One of the most valuable aspects of applications built on Flux is that their components become incredibly easy to test. Developers can recreate and test the state of any React component by simply updating the store — direct interactions with the UI (with tools like Selenium) are no longer necessary in many cases.

Popular Flux Libraries

While Flux is a general pattern for enforcing data flow through an application, there exist many implementations from which to choose. There are nuances between each implementation, as well as specific pros and cons to consider. The candidate should provide examples of real-world experience with using Flux.

1. What are portals in React?

Most applications contain modals, toasts, and other elements that require escaping the default DOM hierarchy. Portals provide a convenient way to render components in a separate DOM node. The candidate should be able to explain how portals work and their ideal use cases.

Portal is React’s recommended way to render children components into a DOM node that exists outside the DOM hierarchy of the parent component.

A portal is created by calling ReactDom ‘s createPortal function.

The createPortal function accepts two parameters:

content – a valid renderable React element
containerElement – a valid DOM element to append the content

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2. What are error boundaries ?

A JavaScript error in one part of the app shouldn’t break the entire app. Error boundary is React’s solution to this common problem. The candidate should be able to explain an error boundary and use it to handle errors in your application.

Error boundaries are React components that catch JavaScript errors anywhere in its child component tree. The Error boundary component can then log those errors and display fallback UI instead of crashing the entire component tree. You can think of error boundaries as a catch block for components.

The snippet below shows a simple ErrorBoundary component that displays a fallback h1 and logs the error.

The ErrorBoundary component can then be used to catch all errors in the Child component as follows:

3. What is React Profiler and what is it used for?

Optimizations are a key part of every application. Knowing which tools to use can be a huge productivity boost. This is a more advanced question that you might ask to understand the candidate’s ability to diagnose and optimize a React application.

React Profiler is a means to measure the cost of rendering in a React application. The purpose of this component is to help developers identify parts of the application that are slow and may benefit from further optimizations.

The Profiler can be added anywhere in the component tree to measure its rendering cost. For example, the code below shows how the Profiler component is used to measure the rendering cost of the Navigation component and its descendants:

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4. What is StrictMode in React?

Catching potential problems is a huge benefit when building applications. React has a built-in component to help developers do just that. You might ask your candidate this question to better understand the depth of their knowledge of more advanced React components. The candidate should be able to explain what the component does, its usage, and its benefits.

StrictMode is a tool to highlight potential problems in an application. Although StrictMode is used as a component, it doesn’t create a visible UI in the DOM. It only enables additional checks for its descendants.

You can enable strict mode for any part of your application by wrapping the components inside the React.StrictMode component. The following code shows how strict mode checks are run on the BlogContent component and its descendants. The Header component won’t be checked as it is outside of the React.StrictMode component.

Benefits of using StrictMode include:

Identifying components with unsafe lifecycles
Warning about legacy string ref API usage
Warning about deprecated findDOMNode usage
Detecting unexpected side effects
Detecting legacy context API
Ensuring reusable state

5. What are React Fragments used for?

Sometimes, we need to return multiple React elements from a single parent. This only works when the elements are wrapped under a single generic HTML element like a div . Fragments allow us to return multiple elements directly without the additional wrapper element.

You might ask this question to gauge the candidate’s level of understanding of React components. The candidate should be able to explain what fragments are and the problem they solve.

React fragments are special features that allow creating a group of children elements or components without creating an actual node in the template. Fragments are denoted by an open empty tag ( <> ) and a closed empty tag ( </> ).

The code below shows how a React component could return two td elements without wrapping them inside a div container.

Wrapping Up

Interviewing a React developer involves much more than just testing for React knowledge — you should also ask questions about JavaScript and about other nuances more closely related to the project or job in question.

This article attempted to cover some basic talking points that would demonstrate whether or not a developer has an adequate understanding of React and its core concepts. I hope you find it useful — good luck!

You can also explore HireAI to skip the line and:

⚡️ Get instant candidate matches without searching ⚡️ Identify top applicants from our network of 250,000+ devs with no manual screening ⚡️ Hire 4x faster with vetted candidates (qualified and interview-ready)

This article was written by William Juan, with elements grandfathered in from an earlier article by Arthur Kay. Arthur is a developer who has been working with the Web since the late 1990s, when GeoCities and scrolling marquees were all the rage. Arthur graduated from Loyola University Chicago and now works as a Senior Software Engineer with a core focus on JavaScript.

William Juan

Web & Mobile Front-End Developer

William is a front-end developer working primarily in the web and hybrid mobile spaces. The majority of his work has revolved around the Angular ecosystem, including working with other Angular-related frameworks such as NativeScript and Ionic. At Arc, he contributes the expertise he's gained over years as a writer on software development careers.

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Posted on May 2, 2023

Crush Your React and Redux Interview: 20+ Top Questions and Strategies

1. what is react, 2. what do you like and dislike about react, 3. what do you understand by conditional rendering and list rendering, 4. what is the significance of having the key prop when rendering a list of items in react, 5. what is the potential bug you can introduce when using index of an array as a key, 6. what is prop drilling and how can you avoid that using the react context api, 7. what was the need for react hooks, 8. explain the usage of usestate, useeffect and usecontext, 9. how do you optimize a react application, 10. what are pure components in react, 11. what is react memo, 12. what is usememo and usecallback in react outline their differences., 13. how do you share logic across components in react, 14. what are some of the packages you use along with react, 15. what is react virtual dom and differentiate it from the real dom, 16. what is redux, 17. how do you decide whether to choose react context api or redux, 18. what is your understanding of redux store, actions, action creators and reducers, 19. what is the control flow between the redux store, actions, action creators and reducers, 20. what does the connect function do from the react-redux library, 21. why should you dispatch an action to update the state and modify it directly.

React is one of the most popular front-end frameworks in the world, and as a result, React interviews are highly competitive. Whether you're an experienced React developer or just starting out, preparing for an interview can be daunting. In this post, I'll provide you with the top questions and strategies to help you crush your React interview. By the end of this post, you'll have the confidence and knowledge you need to ace any React interview and land your dream job. So, let's dive in and get you ready to impress!

🚧 This is not an exhaustive list by any means but it's a list you should definitely be ready for 🚧

React Questions

Typically, the first question in a React interview is about React's definition. However, it's crucial that your response goes beyond just defining React. To demonstrate your comprehension of React, it's essential to explain it in a straightforward manner.

👇👇 React is a popular JavaScript library used for building user interfaces (UIs) in web applications. It was developed by Facebook and is now maintained by a community of developers. React uses a component-based approach, where each UI element is broken down into smaller, reusable components that can be easily managed and manipulated.

One of the main advantages of React is that it allows developers to write code in a declarative way, meaning they can describe what the UI should look like at any given point in time, and React takes care of updating the actual UI to reflect those changes. This makes it much easier to build complex, interactive UIs that respond quickly to user input.

React also has a number of other features that make it a powerful tool for web development, including virtual DOM (Document Object Model) manipulation, server-side rendering, and the ability to easily integrate with other libraries and frameworks. It is often used in conjunction with other front-end technologies such as Redux for state management, and React Native for building mobile applications.

Overall, React is a flexible and powerful tool for building complex and dynamic user interfaces, and it is widely used in the web development community.

So, when you're in a React interview and the interviewer asks, "What do you like and dislike about React?" they want to know your thoughts and experience with the React library. They're basically trying to see if you can evaluate technology and how familiar you are with it.

The interviewer also wants to know how interested and excited you are about React. Your answer can show whether you're genuinely passionate about it or if you're just using it because it's trendy right now.

I like the unidirectional flow parent to child and the fact that React is pretty much JavaScript. Since JavaScript is at the heart of web development, React makes building web apps a lot more enjoyable and straightforward.
An additional reason why I appreciate React is its ease of adoption. Through my experience working on existing codebases, I have found React code to be relatively comprehensible, making it a promising technology for new projects or team members who are not yet familiar with the codebase.
I struggled initially to grasp React Server Components (RSC) and JSX, and I found that to be a bit frustrating. However, despite this, I still consider React to be my preferred library for building user interfaces.

Conditional rendering and list rendering are two really useful concepts in React that let you show different things on your web page depending on what's happening or what data you have.

Conditional rendering means you can decide whether to show one thing or another based on certain conditions. For example, if someone is logged in to your site, you might show them a "log out" button, but if they're not logged in, you might show them a "log in" button instead.

List rendering is all about showing a bunch of things on your page that are all based on the same kind of data. You can use it to show a list of blog posts, for example, or a list of products for sale.

Both of these techniques are really important for making your web page more dynamic and interactive. By using them, you can make your site respond in real-time to user actions and data changes.

When you're using React to show a list of items on your web page, it's really important to include a "key" prop for each item in the list.

Basically, the "key" prop is a way for React to keep track of which item is which in your list. It helps React to know which items have been added, removed, or updated when you change the data in your list.

Without the "key" prop, React might get confused and not update your list correctly. For example, if you add a new item to your list, React might think that all the other items have changed too, and it might try to re-render the whole list, which could make your page slow down or even crash!

So, including a "key" prop for each item in your list is really important if you want your web page to be fast, responsive, and reliable. It's a small thing, but it can make a big difference!

In React, when you're rendering a list of items, you need to give each item a unique "key" prop. This helps React to keep track of which item is which and update the list correctly when you change the data.

Well, it turns out that using the index as the "key" prop can actually introduce a potential bug. Here's why:

Let's say you have a list of items, and you remove the first item from the list. Now, all the other items shift down by one position in the array, so their indexes change. But if you're using the index as the "key" prop, React will still think that each item has the same key as before. This can cause React to get confused and not update the list correctly, leading to bugs and unexpected behavior.

So, the bottom line is: don't use the index of the array as the "key" prop in React! Instead, try to use a unique identifier for each item, like a database ID or some other value that won't change if the data changes. This will help ensure that your list rendering is reliable and bug-free.

Prop drilling is a common problem in React that happens when you have to pass data down through several layers of components, even if some of those components don't need that data. This can make your code messy and hard to manage, and it's often called "prop drilling" because you end up drilling the same props down through multiple layers of your component tree.

The context API lets you pass data down through your component tree without having to drill it through every layer manually. Instead, you can define a "context" object at a higher level in your component tree, and then any component below that level can access that context object without having to pass it down through props.

This can make your code much cleaner and easier to manage, because you don't have to worry about passing props through layers of components that don't need them.

To use the context API, you define a "provider" component that creates a new context object and sets its value to whatever data you want to pass down. Then, any component below that provider in the tree can access the context object using a "consumer" component.

So, the bottom line is: if you find yourself prop drilling in React, consider using the context API to avoid that mess! It can make your code more organized and easier to work with.

Before React Hooks came along, managing state and lifecycle methods in React could be a bit messy and confusing. You had to use class components to manage state and lifecycle, which could make your code verbose and hard to understand.

React Hooks were introduced to make it easier and more intuitive to manage state and lifecycle in React functional components. Hooks are essentially functions that let you "hook into" React's state and lifecycle features, without needing to use class components.

With hooks, you can now use state and lifecycle methods in functional components just as easily as you could in class components, making your code more concise and readable. Plus, hooks give you new capabilities that weren't available before, such as the ability to use state in custom hooks and reuse logic across multiple components.

In short, React Hooks were created to simplify the process of managing state and lifecycle in React functional components, and to give developers more flexibility and power in how they manage their React applications.

useState, useEffect, and useContext are three important React hooks that are frequently used in React development.

useState is a hook that lets you add state to your functional components. You call useState with an initial value, and it returns an array with two values: the current state value, and a function to update that state value. You can use this state value to store data and update it as needed, just like you would with class components.

useEffect is a hook that lets you perform side effects in your components. Side effects are actions that don't directly affect the UI, but might involve things like fetching data from a server, updating the document title, or setting up event listeners. You can use useEffect to run code in response to changes in your component's state or props, or to run code just once when the component mounts or unmounts.

useContext is a hook that lets you access a context object from any component in your component tree, without having to pass that object down through props. You can create a context object using the createContext function, and then use the useContext hook to access that object's value in any component that needs it.

These three hooks are essential for building complex, data-driven React applications!

Optimizing a React application involves improving its performance, reducing load times, and making it more efficient. There are several techniques and tools you can use to optimize a React app:

Code splitting: This technique involves breaking your app's code into smaller chunks that can be loaded on demand, rather than all at once. This can improve your app's load time and performance.

Memoization: This technique involves caching the results of expensive function calls, so that they don't have to be recomputed every time the component renders. This can improve your app's rendering performance and reduce its CPU usage.

Server-side rendering: This technique involves rendering your app's initial HTML on the server, before sending it to the client. This can improve your app's load time and SEO, and also reduce its initial data transfer size.

Performance profiling: This involves using tools like the React Profiler or Chrome DevTools to identify performance bottlenecks in your app, and then optimizing those areas for better performance.

Lazy loading: This technique involves loading components or data on demand, rather than all at once. This can reduce your app's initial load time and improve its performance.

Using a state management library: If your app has complex state management requirements, using a library like Redux or MobX can help you manage that state more efficiently and improve your app's performance.

In addition, the key is to identify areas where your app is slow or inefficient, and then apply the appropriate optimization techniques to improve its performance and user experience.

In React, pure components are components that only render based on their props and state. They don't rely on any external factors or internal state changes to decide whether to re-render.

Pure components are also known as "dumb" components, because they don't have any logic or side effects of their own. They simply receive data as props and render it to the screen.

The main advantage of using pure components is that they can improve your app's performance, because they only re-render when their props or state change. This can help reduce unnecessary re-renders and improve your app's overall efficiency.

To create a pure component in React, you can either extend the React.PureComponent class, or use the React.memo higher-order component. React.memo is a function that accepts a component and returns a new component that only re-renders when its props change. You can use this function to optimize your components and reduce unnecessary re-renders.

React memo is a higher-order component in React that helps optimize the performance of your components by reducing unnecessary re-renders. It works by caching the result of a component's rendering, and only re-rendering it when its props change.

React memo is similar to React.PureComponent, but it's a functional component that accepts another component as its argument, and returns a new component that is optimized for performance.

To use React memo, you can wrap your component in the memo function and pass it as an argument. For example:

In React, useMemo and useCallback are two hooks that can help optimize the performance of your components by memoizing expensive function calls.

useMemo is a hook that memoizes the result of a function call, and only re-runs that function if its dependencies change. This can be useful for expensive calculations or operations that are used in your component's rendering logic.

Here's an example of using useMemo to memoize an expensive calculation:

useCallback, on the other hand, is a hook that memoizes a function definition, and only re-creates that function if its dependencies change. This can be useful for optimizing child components that rely on callbacks from a parent component.

Here's an example of using useCallback to memoize a function:

The main difference between useMemo and useCallback is that useMemo memoizes the result of a function call, while useCallback memoizes the function definition itself. Additionally, useCallback is usually used for optimizing child components that rely on callbacks from a parent component, while useMemo is more commonly used for expensive calculations or operations that are used in your component's rendering logic.

This is a question that can assist the interviewer in gauging the level of reusability in your React code.

Sharing logic across components in React can be accomplished in a few different ways. Here are a few common approaches:

Higher-order components (HOCs): A higher-order component is a function that takes a component as an argument and returns a new component with additional functionality. You can use HOCs to share common functionality across multiple components without duplicating code.

Render props: A render prop is a function that a component uses to share its state or functionality with another component. The component with the render prop provides a function as a prop, and the component that uses it can call that function to access the shared logic.

Custom hooks: A custom hook is a function that encapsulates reusable logic that can be shared across multiple components. Custom hooks can be used to abstract away complex logic and make it easier to reuse in different parts of your application.

Here's an example of how you might use a custom hook to share logic across multiple components:

When working with React, there are a number of packages that can be helpful to include in your project. Here are a few examples:

React Router: A package that provides routing capabilities to a React application, allowing you to navigate between different pages or views.

Redux: A state management library that helps manage complex application state in a predictable way, making it easier to reason about and debug.

Axios: A package that provides an easy-to-use interface for making HTTP requests, making it simple to communicate with a server or API.

Formik: A package that simplifies the process of building and validating forms in React, making it easier to handle user input.

Styled Components: A package that allows you to write CSS in your JavaScript code, making it easier to style components in a reusable and maintainable way.

React Native: A framework for building native mobile apps using React, allowing you to use your existing React knowledge to build iOS and Android apps.

There are many other packages available that can be useful when working with React. The specific packages you choose to use will depend on your specific use case and the requirements of your project.

In React, the virtual DOM is a lightweight representation of the actual DOM (Document Object Model) that the browser uses to render web pages. The virtual DOM is an abstraction layer that allows React to update the UI more efficiently, without directly manipulating the real DOM.

Here are a few key differences between the virtual DOM and the real DOM:

Performance: The virtual DOM is faster than the real DOM because it reduces the number of updates needed to keep the UI in sync with the application state. When an update occurs, React compares the new virtual DOM tree with the old one and calculates the minimal set of changes needed to update the real DOM. This is much faster than updating the entire real DOM tree.

Manipulation: The virtual DOM can be manipulated more easily than the real DOM, which can be slow and error-prone. In React, you can update the virtual DOM directly using JSX syntax, which is then converted into a virtual DOM tree. This allows you to focus on the application logic rather than the intricacies of DOM manipulation.

Memory usage: The virtual DOM is less memory-intensive than the real DOM because it only stores a lightweight representation of the DOM tree in memory. This means that React applications can handle large amounts of data and complex UIs without running out of memory.

Rendering: The virtual DOM provides a declarative way to describe the UI, which makes it easier to reason about and test. In React, you declare what the UI should look like, and React takes care of rendering it efficiently to the real DOM.

Redux Questions

Redux is an open-source JavaScript library for managing the state of a web application. It is often used with React, but can also be used with other frameworks or libraries. Redux follows a unidirectional data flow pattern, where all data in the application flows in a single direction. The state of the application is stored in a single store, which is managed by reducers. Actions are dispatched to the store to modify the state, and views are updated accordingly.

One of the main benefits of Redux is that it makes it easier to manage complex application state in a predictable way, making it easier to reason about and debug. It also allows for better separation of concerns, as the state management logic is decoupled from the UI components.

Redux can be used with a variety of front-end and back-end technologies, and has a large ecosystem of extensions and middleware that can be used to add additional functionality. However, it can add additional complexity to a project, and may not be necessary for smaller applications with simpler state management needs.

Deciding whether to use React Context API or Redux in a project can depend on a number of factors. Here are some things to consider:

Size and complexity of the application: If the application is relatively small or simple, it may not require the additional complexity of Redux. React Context API may be a simpler and more lightweight solution for managing state.

Data sharing needs: If the data in the application needs to be shared across multiple components that are not directly related to each other, Redux may be a better fit. React Context API is better suited for sharing data between components that are closely related to each other, such as parent and child components.

Time and resources: If you have limited time or resources, it may be more efficient to use React Context API, as it is built into React and requires less setup than Redux. However, if you have the time and resources to invest in setting up Redux, it can provide more advanced features and better scalability in the long run.

Familiarity with Redux: If you or your team are already familiar with Redux and have experience using it, it may be easier to use Redux in your project. On the other hand, if you or your team are new to Redux, it may be more efficient to use React Context API.

The decision of whether to use React Context API or Redux will depend on the specific needs of your project, as well as your own preferences and experience. It may be helpful to prototype the application using both approaches to determine which one works best for your specific use case.

In Redux, the state of the application is stored in a single object called the store . The store is managed by reducers, which are functions that specify how the state should be modified in response to actions.

Actions are plain JavaScript objects that describe what happened in the application. They typically have a type property that indicates the type of action that occurred, as well as any additional data that is needed to update the state.

Action creators are functions that create and return action objects. They are often used to encapsulate the logic for creating actions and can also perform additional processing before returning the action.

Reducers are functions that take the current state and an action as input, and return a new state object as output. Reducers are responsible for updating the state in response to actions, and should not modify the original state object directly.

Together, they form the core of the Redux architecture and provide a predictable and scalable way to manage state in a web application.

In a typical Redux application, the control flow between the Redux store, actions, action creators, and reducers is as follows:

The application triggers an action by calling an action creator function. The action creator function returns a plain JavaScript object that represents the action and includes a type property that describes the type of action that occurred.

The Redux store receives the action and passes it to the root reducer.

The root reducer delegates the action to one or more child reducers, each of which is responsible for updating a specific part of the application state.

The child reducer updates its part of the state based on the action, and returns a new state object.

The root reducer combines the updated state from each child reducer into a single object, and returns the new state object to the Redux store.

The Redux store updates its state with the new state object, and notifies any connected components that the state has changed.

The connected components retrieve the updated state from the Redux store, and re-render as needed to reflect the new state.

The connect function from the react-redux library is used to connect a React component to the Redux store. It is a higher-order component that takes in two arguments: mapStateToProps and mapDispatchToProps .

The mapStateToProps function is used to map state from the Redux store to the props of the connected component. It takes in the current state of the store and returns an object that defines the props that should be passed to the connected component. When the state of the store changes, the connected component is automatically re-rendered with the updated props.

The mapDispatchToProps function is used to map action creators from Redux to the props of the connected component. It takes in the dispatch function as an argument, which can be used to dispatch actions to the store. The mapDispatchToProps function returns an object that defines the props that should be passed to the connected component. When an action is dispatched, the connected component is automatically re-rendered with the updated props.

Together, these two functions allow a React component to interact with the Redux store and dispatch actions to update the state. The connect function also provides additional optimizations, such as preventing unnecessary re-renders of the connected component, and ensuring that the component only re-renders when the relevant state or props have changed.

In a Redux application, you should always dispatch an action to update the state, rather than modifying it directly. Here are a few reasons why:

Predictable state updates: By dispatching an action, you ensure that the state is updated in a predictable and consistent way. The action contains a description of what happened, which makes it easier to debug and understand how the state changed over time.

Time travel debugging: When you dispatch an action, Redux records it in the store's history. This allows you to use tools like the Redux DevTools to "time travel" and inspect the state of the application at any point in time, making it easier to debug and identify issues.

Middleware and side effects: Dispatching an action allows you to use middleware, which can intercept and modify the action before it reaches the reducers. This is useful for handling side effects, such as making network requests or interacting with APIs.

Encapsulation and modularity: By dispatching an action, you ensure that state updates are encapsulated and modular. Each action represents a discrete change to the state, making it easier to reason about and test.

On the other hand, modifying the state directly can lead to unpredictable behavior and bugs, especially as the application grows and becomes more complex. It also makes it harder to debug and understand how the state changed over time. By dispatching actions to update the state, you ensure that the application remains predictable, testable, and maintainable over time.

🚧🚧🚧 NEVER MODIFY THE STATE DIRECTLY 🚧🚧🚧

It's normal to feel nervous before a job interview, but it's important to remember that you were selected for an interview because the employer saw something in your application that impressed them. So, use the interview as an opportunity to showcase your skills and knowledge, and to demonstrate why you would be a great fit for the position. Be confident, enthusiastic, and honest about your strengths and weaknesses. Remember to listen attentively to the interviewer, and ask thoughtful questions that show your interest in the company and the role. And above all, be yourself - your unique experiences, skills, and personality are what make you stand out from other candidates. Good luck with your interview!

Hey folks! We've come to the end of this discussion. If you have any questions or thoughts, feel free to drop them in the comments section below. And hey, don't forget to follow me on Twitter .

Also, if you're interested, I could write a detailed tutorial on Redux. Let me know, and I'd be more than happy to do that for you. Cheers!

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10 React Interview Questions to Practice

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React is an open-source JavaScript framework developed by Facebook. It’s currently the most popular front-end framework in use today. Its success even convinced Facebook to develop React Native , another framework that uses React for cross-platform mobile development .

When you’re interviewing for a job that requires using React daily, employers want to know that you have the skills needed to get the job done. That means your interview will likely include React interview questions, and while we can’t guess precisely what they’ll ask, we’ve collected some of the most popular ones here, along with possible answers.

1. What is React, and how is it different from other JavaScript frameworks?

With this question, the interviewer wants to find out if you know about the JavaScript ecosystem in general and what makes React unique. One thing you could bring up is that while some developers call React a framework , many consider it a library and not a full framework.

This is because React’s purpose is to build UI components for your web app. That’s all it really does. It’s the V (or View) part of an MVC (Model, View, Controller) architecture , and to build a complete application with React, you need other libraries like Redux .

Most JavaScript frameworks, like AngularJS , have tools for creating controllers, directives, services, and other functionalities. With these other frameworks, you have to use the tools you’re given for the most part. With React, you can build an application with the libraries of your choice.

2. What is JSX?

JSX is a language that React uses to embed an HTML template in code. It isn’t understood by browsers and must be transpiled with tools like webpack and Babel before it can be run.

Still, JSX helps streamline React development once you know its syntax. You don’t actually have to use JSX to create React components and can use React.createElement() instead, but Facebook recommends using JSX. Most React developers prefer its declarative syntax, and it also reduces code complexity.

Here’s an example of JSX:

3. What is state in React?

State is a JavaScript object used to represent the data held in a React app that may change over time. For example, you might have a state variable called clicks which updates when a button is pressed.

State is fully contained within the component, unlike props. In class components, you use the state class variable to set a state variable, like in the example below:

In functional components, you can set state in the component with the useState hook:

4. What are props in React?

Props are inputs for components and a way for passing data from one component to a child component. They can be single values, objects, or functions. Here’s an example:

In the example above, the MyHeader component is passed the greeting prop and can be accessed inside that component with props.greeting .

5. What are the three main phases of a React component’s lifecycle?

The initial rendering phase , when the component is about to modify the DOM of the web page.
The updating phase after the component is added to the DOM and only updates when its state or props change.
The unmounting phase , when the component is destroyed and removed from the DOM.

6. What are synthetic events in React?

React wraps the native events of the browser with its own event structure for cross-browser compatibility. It has the same API as the browser’s events but acts the same in every browser.

7. Where is using a key prop necessary, and why?

Key props are required when you create an array of elements. React uses this key to identify specific elements in the array when they’re updated, removed, or added.

8. What is a ref in React?

A ref (reference) in React is an attribute that gives you a handle to access the functions in a specific element. Here’s an example:

The code above will focus the cursor in the input using a ref .

9. What are Higher-Order Components (HOC)?

Higher-Order Components are custom components that wrap other components within them. They can dynamically accept one or more components as children without modifying or copying any of the children’s behavior. They allow for code reuse and state abstraction and manipulation.

10. What are pure components?

Pure components in React don’t re-render when props or states update to the same value. You can extend the React.PureComponent class to use them. The rendering is restricted in this way for higher performance and is often used for components that are mainly for display.

Learn more about React

Don’t worry about preparing for every possible question you might face during your interview. That’d be impossible. Instead, focus on solidifying your understanding of React concepts and functions, and if you need a refresher, try courses like:

Learn React
Create a Front-End App With React

But remember, even if the company you’re interviewing with relies heavily on React, they’ll likely test your knowledge of other programming tools and functions. You might even be asked to complete a technical interview. For more information on interviewing in the tech industry, check out the articles below. Then, for more tips on how to prepare, visit our Career Center .

Interviewing in the tech industry: Technical and behavioral interview tips
Acing the technical interview: A complete guide

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Acing the ReactJS Interview: Common Questions and Sample Answers

Prepare for your ReactJS interview with our detailed Q&A. Understand Virtual DOM, component lifecycle, state management, and more for interview success.

Avinash Bidkar

Introduction.

In the dynamic landscape of modern web development, ReactJS has emerged as a cornerstone technology, revolutionizing how user interfaces are built and maintained. As React gains prominence, so does the demand for skilled ReactJS developers who can harness its power to craft seamless, interactive web applications. The ReactJS interview process is a pivotal gateway to these opportunities, testing candidates on their theoretical knowledge, coding prowess, and problem-solving acumen. This blog is your ultimate guide to facing and acing your ReactJS interview.

Common ReactJS Interview Questions and Sample Answers

Here are some common and frequently asked ReactJS interview questions and their answers.

1. What is ReactJS, and how does it differ from other JavaScript frameworks?

ReactJS is an open-source JavaScript library designed for building user interfaces. Unlike traditional frameworks, React focuses solely on the view layer, making it highly efficient for creating dynamic UIs. Its component-based architecture promotes reusability and modularity, allowing developers to compose complex interfaces from smaller, self-contained building blocks. This contrasts with frameworks like Angular, which offer a more comprehensive structure encompassing data management and routing. Additionally, React's virtual DOM optimizes rendering, updating only the necessary parts of the actual DOM, which enhances performance compared to other frameworks that might re-render entire sections.

2. Explain the concept of JSX.

JSX, or JavaScript XML, is a syntax extension used in React to describe the structure of UI components. It resembles HTML but seamlessly integrates JavaScript expressions. JSX simplifies component rendering by allowing developers to write expressive, declarative code. Under the hood, JSX gets transpiled into standard JavaScript using tools like Babel. For instance, <Button color="blue" /> in JSX corresponds to React.createElement(Button, {color: 'blue'}) in JavaScript. This blend of HTML-like syntax with JavaScript logic streamlines UI development and enhances code readability.

3. What is the significance of the virtual DOM?

Virtual DOM is a crucial concept in React that boosts performance and efficiency. It's an abstraction of the actual DOM, a lightweight representation of UI components and their structure. When changes occur, React compares the virtual DOM with the real DOM, computing the minimal necessary updates. This leads to fewer direct manipulations of the actual DOM, reducing rendering overhead and enhancing speed. Consequently, applications built with React deliver a smoother user experience as updates are processed optimally, minimizing unnecessary reflows and repaints.

4. How do React props work?

React props, short for properties, pass data from a parent component to its child components. They enable communication and allow components to be reusable and modular. Props are immutable and provide a way to share information between components without directly modifying the component's state. By defining attributes within JSX tags, you can pass data down the component hierarchy. For instance, <UserProfile name="John" age={25} /> sends the name and age values as props to the UserProfile component. Inside the component, these values are accessible as this.props.name and this.props.age.

5. What is React state, and how is it different from props?

React state is a built-in feature that allows components to store and manage their internal data. While similar to props, which are passed from parent to child components, the state is local and mutable, whereas props are immutable. The state is primarily used for dynamic data, like user input or component-specific values, that can change over time. Unlike props, which are externally controlled, a component manages its state. Using the setState method, you can modify the state, prompting React to re-render the component and update the UI based on the changes.

6. Explain the component lifecycle in React.

The React component lifecycle consists of three main phases: mounting, updating, and unmounting. The component is initialized during mounting, and methods like constructor and componentDidMount are executed. In the updating phase, changes trigger methods like shouldComponentUpdate to determine if re-rendering is needed, followed by componentDidUpdate after rendering. Finally, in the unmounting phase, the component is removed from the DOM, and the component will unmount method is called. Understanding these phases helps manage component behavior, optimize rendering, and perform cleanup tasks.

7. How does React handle forms?

React provides two approaches for handling forms: controlled components and uncontrolled components. Controlled components maintain form data in the component's state, enabling dynamic updates and validation. Uncontrolled components, on the other hand, rely on the DOM for form data storage. Developers can access form values using references after user input. Controlled components offer better control and validation, making them a preferred choice for complex forms, while uncontrolled components are suitable for simpler scenarios.

8. What is the significance of keys in React lists?

Keys are essential when rendering lists of elements in React. They provide a unique identifier for each item in a list, enabling React to track and manage updates efficiently. When items are added, removed, or rearranged, keys ensure that only the necessary elements are modified in the DOM. React might re-render the entire list without keys, resulting in performance issues and unintended side effects. Assigning a key to each list item ensures stable and predictable rendering behavior.

9. How can you optimize a React application's performance?

Optimizing a React application's performance involves techniques like code splitting, lazy loading, and memoization. Code splitting divides the application into smaller chunks, loaded on-demand, reducing the initial load time. Lazy loading defers loading non-essential components until they are needed. Memoization optimizes rendering by caching the results of expensive function calls using React. memo and useMemo, preventing unnecessary recalculations.

10. Describe the concept of "lifting state."

"Lifting state up" is a pattern in React where the shared state is managed at a higher level in the component hierarchy and passed down as props to child components. This pattern enhances data consistency and synchronization between components. By centralizing state management, you prevent inconsistencies and improve maintainability. For instance, when multiple child components need access to the same data, lifting the state to a common ancestor simplifies data management, reduces redundancy, and enhances component interaction.

In the ever-evolving realm of web development, ReactJS is a beacon of innovation, shaping the digital landscape with its dynamic capabilities. As you embark on your ReactJS interview journey , remember that success goes beyond mastering technical concepts – it encompasses effective communication and the ability to showcase your skills with clarity. By embracing the insights shared in this guide, you can navigate the diverse ReactJS interview questions confidently. Your journey to ReactJS excellence starts here. Best of luck!

What should I anticipate from a ReactJS job interview?

Your theoretical knowledge, technical skills, and problem-solving talents are evaluated during ReactJS interviews. React principles, JSX, virtual DOM, state management, and component lifecycle are topics that interviewers frequently probe.

What distinguishes ReactJS from other JavaScript frameworks?

A JavaScript library devoted to creating user interfaces is called ReactJS. It excels at building dynamic user interfaces by quickly drawing just the required bits of the view. React's component-based architecture, in contrast, to complete frameworks like Angular, encourages reusability and modularity, making it a popular option for front-end development.

Can you describe JSX and how it is used to create React?

React uses the grammar extension known as JSX, or JavaScript XML, to express UI components. It simplifies component rendering and improves code readability by combining JavaScript expressions with syntax similar to HTML. The declarative nature of React programming is enhanced by translating JSX into regular JavaScript code.

In React, what role does the virtual DOM play?

React's virtual DOM is a key idea that improves performance. It optimizes UI rendering by computing the fewest updates possible and is a lightweight approximation of the actual DOM.

How are props and React states different?

React state is mutable and used to manage internal data within components. Props, on the other hand, are immutable and handed from parent to child components. Props enable communication between components without directly affecting their internal state, whereas the state is appropriate for maintaining dynamic data that might change over time

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About our React coding tests

We provide React code exams for your team that are realistic and useful, giving a precise assessment of candidates’ React skills. Even if you don’t have a technical background, you can develop React code tests in under a minute using tools like CoderPad Screen.

We have created our tests to precisely cover important ideas like object-oriented programming, data structures, algorithms, and more. Our industry specialists have carefully selected them to ensure their applicability to real-world situations and their efficacy in evaluating candidates’ talents.

There is an ability to modify the tests and add their own coding activities for hiring managers who wish to go further.

Recommended duration: 20-60 min
Average number of questions: 20
Type of exercises: Multiple choice, coding exercises
Test levels : Junior, Senior, Expert

ℹ️ Explore our questions bank and test our candidate experience yourself

React skills to assess

Component lifecycle
React hooks
React context API
State management

Jobs using React

Front-End developer

React sample questions

Example question 1.

What is the difference between a class component and a functional component in React? What is the difference between a class component and a functional component in React?

Example Question 2

Your task is to create a working todo list with persistent data storage.

Requirements

Clicking on a todo item should toggle the “checked” state.
The todo list state should be saved and loaded from local storage.
Checked items should sink to the bottom of the list automatically

ℹ️ Preview a sample coding test report

Explore Code Playback

Obtain a deeper understanding of the problem solving thought process of your developer applicants.

Witness their proficiency in building algorithms and evaluate their approach to solving problems with them.
Jot down feedback from different segments of the interview to discuss in subsequent interview rounds.
Pay attention to detect and prevent any distasteful actions.

How to test React Skills to hire for your team?

Relying only on resumes may not accurately paint a broad picture of a React developer’s skills and abilities, and self-reported skills may be untrustworthy.

Here are five ways to assess React developers’ coding skills:

Examine their body of work. This provides you with more information about the kinds of React projects they have worked on and their level of language expertise.
Examine their GitHub account to judge the quality of their React code, their projects, and their participation in React-related projects.
Ask about their use of Stack Overflow or React communities in their day-to-day development to understand their knowledge level and participation in the React development community.
Employ the use of practical React focused programming exercises for React developers to allow them assess and improve their coding skills.
Conduct React-focused live coding interviews in which candidates can exhibit their coding skills, problem-solving talents, and understanding of React fundamentals.

💡 To dig deeper: 5 ways to test developers’ skills before hiring

How to approach initial candidate screening effectively?

Some recruiters are concerned that putting coding tests on candidates early in the interview process will intimidate potential prospects and discourage applications. However, good scenario management, such as clarifying the interview process and why it is vital, will help candidates comprehend the goal and feel at ease.

Here are some guidelines for you to ensure a positive candidate experience during early testing:

Job Ad : Make certain that the advertisement includes every phase and expectation of the hiring process. As a result, the candidate will not feel singled out and will be able to fully prepare for the interview. It is also a good idea to convey the time limits involved in technical interviews ahead of time.
Highlight Equal Opportunities and Diversity: Emphasize your company’s commitment to promote diversity and equal opportunity through the testing process. It must to be obvious that the major objective of the assessment is to fairly evaluate the candidate talents, regardless of history.
Concise Assessment: Choose quality over quantity by keeping the assessment brief and focused on the skills required for the job. This is to eliminate any time wasting and ensure the candidates can attempt the exercise hitch-free. Ideally, aim for a test that can be completed within an hour or less without compromising any requirements.
Relevance: By ensuring that the examinations only focus on the abilities that are specifically relevant to the position, redundancies can be avoided. Developers value candidates that show pragmatism by emphasizing practical abilities that have an impact on daily work.
Feedback and Results : Provide applicants with performance comments after the testing phases. You may improve candidates’ experiences by using a product like CoderPad Screen, which automatically creates a concise report and sends it to them. Candidates who do poorly will be able to understand why they were passed over for the following round in this way.

We had no unified approach or tool to assess technical ability. Today, CoderPad Screen enables us to test against a wide selection of languages and is continually updated.

1,000 Companies use CoderPad to Screen and Interview Developers

React Interview Mastery: 60 Expert Questions and In-Depth Answers

Elevate your react interview preparation with our extensive library of 60 react questions and detailed expert responses. dive deep into react concepts with insights from seasoned professionals, ensuring you're fully prepared to excel in your react interview. don't miss this opportunity to access top-tier resources that will help you shine in your react interview and propel your career to new heights..

React Basics

1. What is React?

2. What are the key features of React?

Component-Based Architecture : React encourages the creation of reusable UI components. Components are self-contained units of code that can be composed together to build complex user interfaces. This promotes a modular and maintainable code structure.
Virtual DOM (Document Object Model) : React uses a virtual representation of the DOM to optimize rendering. It calculates the difference between the virtual DOM and the actual DOM, minimizing the number of real DOM manipulations. This results in improved performance and responsiveness.
Declarative Syntax : React uses a declarative syntax, allowing developers to describe how the UI should look based on the application's state. You specify what the UI should look like, and React takes care of updating the DOM to match that description. This simplifies UI development and makes it easier to reason about.
Unidirectional Data Flow : React enforces a unidirectional data flow, where data flows in a single direction, typically from parent components to child components. This makes it easier to understand how data changes affect the UI, enhancing predictability and maintainability.
Reusability and Composition : React promotes code reusability by encouraging the creation of small, reusable components. These components can be easily composed to build more complex UIs. This leads to a more efficient development process and reduces code duplication.
React Native : React can be used for mobile app development through React Native. It allows developers to build native mobile apps for multiple platforms, including iOS, Android, and more, using React's component-based approach. Code can be shared between web and mobile applications, reducing development effort.
One-Way Binding : React implements one-way data binding, which simplifies the tracking of data changes. Data flows from parent components to child components, and changes in child components do not directly affect parent components. This clear data flow helps prevent bugs and unexpected behavior.
Rich Ecosystem : React has a thriving ecosystem with a vast collection of libraries, tools, and community-contributed components. This includes state management solutions like Redux and Mobx, routing libraries like React Router, and various UI component libraries.
Server-Side Rendering (SSR) : React supports server-side rendering, which enables improved SEO (Search Engine Optimization) and faster initial page loads. SSR allows rendering React components on the server before sending the HTML to the client.
Community and Support : React has a large and active community, which means that developers have access to a wealth of resources, tutorials, and community support. The community continually contributes to the improvement and growth of the React ecosystem.

3. Explain the virtual DOM in React.

4. How does React differ from other JavaScript frameworks and libraries?

Component-Based : React follows a component-based architecture, allowing developers to create reusable and self-contained UI components. This promotes code reusability and maintainability.
Virtual DOM : React uses a virtual DOM to optimize updates and minimize direct manipulation of the actual DOM, leading to better performance.
Unidirectional Data Flow : React enforces a unidirectional data flow, making it easier to understand and debug data changes within an application.
JSX : React uses JSX (JavaScript XML) for defining component structures in a more declarative and readable way.
Community and Ecosystem : React has a large and active community, along with a rich ecosystem of third-party libraries and tools.

5. What is JSX, and why is it used in React?

6. How do you create a React component?

Functional Component :
Class Component :

7. What is the difference between functional and class components in React?

Function Syntax : Functional components are defined as JavaScript functions.
State Management : Historically, functional components were stateless. However, with the introduction of React Hooks (starting with React 16.8), functional components can now manage state using hooks like useState, useEffect, and useContext. This means you can use state and other React features in functional components.
Lifecycle Methods : Functional components didn't have access to lifecycle methods (e.g., componentDidMount, componentDidUpdate) prior to React 16.8. With hooks like useEffect, you can now replicate the behavior of these lifecycle methods in functional components.
Readability : Functional components are often considered more concise and easier to read than class components.
Performance : Functional components with React Hooks can be optimized for performance using techniques like memoization and useCallback.
Class Syntax : Class components are defined as ES6 classes that extend the React.Component class.
State Management : Class components can manage state using the this.state object. They also have lifecycle methods like componentDidMount, componentDidUpdate, etc., for managing side effects.
Complexity : Class components can become complex, especially for components with many lifecycle methods and state properties.
Boilerplate : Class components often require more boilerplate code compared to functional components, which can make the codebase harder to maintain.
Not Recommended for New Code : As of React 16.8, React encourages the use of functional components with hooks for new code because they offer a more straightforward and flexible way to manage component logic.

8. How do you render a React component?

9. What is a state in React?

10. How can you update the state in a React component?

Functional Component with Hooks :

Component Lifecycle

11. Explain the component lifecycle methods in React.

constructor() : Called when an instance of the component is being created.
render() : Determines the component's output.
componentDidMount() : Invoked after the component is rendered in the DOM.
shouldComponentUpdate() : Decides if the component should re-render or not.
render() : Re-renders the component with updated data.
componentDidUpdate() : Called after the component is re-rendered.
componentWillUnmount() : Triggered just before a component is removed from the DOM.

12. What is the purpose of the componentDidMount method?

13. How can you prevent a component from re-rendering in React?

14. What is the purpose of the shouldComponentUpdate method?

15. Explain the use of componentDidUpdate in React.

16. What is the significance of the componentWillUnmount method?

Props and State

17. What are props in React, and how are they used?

18. How can you pass data from a parent component to a child component in React?

Define the Parent Component : First, create your parent component. This component will be responsible for rendering the child component and passing data to it.
Define the Child Component : Next, create your child component. In this component, you can access the data passed from the parent component through the props object.
Render the Parent Component : Finally, render the parent component in your application's main file (usually index.js or App.js):

19. What is the difference between state and props?

Props are passed from parent to child components.
They are immutable and read-only in the child component.
They are used for data that doesn't change within the component itself.
State is used to manage data that can change within a component.
It is mutable and can be changed using this.setState() method.
State is used for data that needs to be re-rendered and can be changed due to user interactions or other factors.

20. When should you use props, and when should you use state?

21. Can you modify props in a child component?

22. What is prop drilling, and how can you avoid it?

Context API : It allows you to pass data down the component tree without explicitly passing it through each intermediate component.
Redux or Mobx : State management libraries that provide a global store accessible from any component.
Higher-Order Components (HOCs) or Render Props : Techniques for component composition that can encapsulate the logic for passing props.

Hooks Concept

23. What are React Hooks, and why were they introduced?

Easier Reuse of Logic : With class components, it was challenging to share and reuse component logic, such as state and side effects, between different components. Hooks allow you to encapsulate and reuse logic across multiple components, making it easier to maintain and understand your code.
Simplify Complex Components : Hooks help simplify the structure of components by allowing you to use state and side effects directly in functional components. This reduces the need for class components and makes it easier to understand and reason about your code.
Reduce Boilerplate : Class components required a lot of boilerplate code for defining and managing state, handling lifecycle methods, and binding event handlers. Hooks eliminate much of this boilerplate, resulting in cleaner and more concise code.
Improved Readability : Hooks promote a more linear and readable code flow. In class components, logic related to a component could be scattered across various lifecycle methods, making it harder to follow. Hooks encourage colocating related logic in the same function, improving code organization and readability.
Compatibility with Functional Components : React Hooks are compatible with functional components, which are a more modern and concise way of defining React components. This aligns with the trend of writing React components as functions rather than classes.
Better Support for Code Splitting and Optimization : Hooks work well with React's new features like Concurrent Mode and Suspense, making it easier to optimize the performance of your applications.
Gradual Adoption : React Hooks were introduced in a way that allows developers to gradually adopt them in existing codebases without having to rewrite all class components. This makes it easier for teams to transition to Hooks incrementally.
useState : Allows you to add state to functional components.
useEffect : Used for managing side effects in functional components. It replaces lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount.
useContext : Allows you to access a context's value within a functional component.
useReducer : A more advanced way to manage state. It is often used when state logic becomes complex and needs to be handled with predictable state transitions.
useRef : Provides a way to access and interact with the DOM directly or to persist values between renders without causing re-renders.
useMemo and useCallback : These hooks help optimize performance by memoizing values and functions to prevent unnecessary re-computations.
useLayoutEffect : Similar to useEffect, but it fires synchronously after all DOM mutations. Use it for interactions that require synchronous updates.
useDebugValue : A custom hook used for debugging purposes. It allows you to display custom labels for custom hooks in React DevTools.
useEffectOnce : A custom hook that simplifies the use case of running an effect only once when a component mounts.
useHistory, useLocation, and useParams (from react-router-dom) : Hooks provided by the React Router library for managing routing state in your components.
useForm (from libraries like react-hook-form and formik) : Hooks for managing form state, validation, and submission.
useQuery and useMutation (from libraries like react-query and SWR) : Hooks for managing data fetching and mutations in a declarative way.
useAsync (from libraries like react-async) : A hook for managing asynchronous operations with status updates.
useWindowSize (custom hook) : A custom hook for tracking window dimensions and responding to changes.

24. Explain the useState hook in React.

25. How do you use the useEffect hook in React?

26. What is the purpose of the useContext hook?

27. How can you create a custom hook in React?

28. What is the useReducer hook, and when would you use it?

Routing and Navigation

29. How can you implement routing in a React application?

Install React Router : npm install react-router-dom
Set up routes and route components using <Route> and <BrowserRouter> or <HashRouter> as the router component.
Use <Link> or useHistory for navigation.

30. What is React Router, and how does it work?

31. Explain the purpose of route parameters in React Router.

32. How can you handle 404 errors in a React Router application?

33. What is client-side routing, and what are its advantages and disadvantages?

Faster navigation : Since only the content that changes is fetched from the server, navigation is faster.
Smooth user experience : Transitions between pages are seamless, as the entire page isn't reloaded.
Better control : Developers have more control over the routing logic and can implement complex navigation patterns.
Initial load time : SPAs often require more initial JavaScript and CSS to be loaded, which can affect the initial page load time.
SEO challenges : Search engine optimization can be more challenging because search engine crawlers may not execute JavaScript.
Browser history issues : Handling browser history and deep linking can be complex.

Forms in React

34. How do you handle forms in React?

35. What is controlled and uncontrolled components in React forms?

Controlled Component : Controlled components are React form elements whose values are controlled by React's state. In controlled components, you set the value of the form element to a state variable and provide an onChange event handler to update the state when the user interacts with the form.
Uncontrolled Component : Uncontrolled components, on the other hand, are form elements whose values are not controlled by React's state. Instead, they rely on the DOM for their values. You can still access their values using DOM manipulation, but React doesn't manage them directly.

36. Explain the onChange event handler in React forms.

37. How can you perform form validation in React?

Set Up State : Initialize state variables for each form input field, typically using the useState hook. These state variables will store the user's input.
Handle Input Changes : Attach onChange event handlers to the form elements to update the corresponding state variables whenever the user interacts with the form.
Perform Validation : Implement validation logic within the event handlers or separately, depending on your preference. You can use conditional statements to check if the input data meets your validation criteria (e.g., required fields, minimum length, valid email format, etc.).
Display Validation Messages : If validation fails, display error messages to the user indicating what needs to be corrected. These error messages can be conditionally rendered based on the validation results.
Submit Data : When the form is successfully validated, you can submit the data to the server or take any other appropriate action.

38. What is the purpose of the useState hook in form handling?

State Management : useState allows you to create and manage state variables that store the current values of form inputs. When the user interacts with the form (e.g., typing in an input field), the state variables are updated accordingly.
Reactivity : By using state variables to control the values of form elements (controlled components), React automatically re-renders the component whenever the state changes. This ensures that the UI reflects the latest user input.
Validation and Data Handling : State variables can be used to validate user input and prepare data for submission to the server. You can also conditionally render UI elements based on the state, such as displaying error messages when validation fails.
Synchronization : useState ensures that the form values are synchronized with the component's state, allowing you to access and manipulate those values easily.

State Management

39. What is Redux, and how does it work with React?

40. What problem does Redux solve in React applications?

Complex State Management : As React applications grow in complexity, managing state becomes challenging. Redux centralizes the state management, making it easier to organize and maintain.
Component Communication : Redux provides a way for components deep in the component tree to access and update shared state without passing data through props manually.
Predictable State Changes : Redux enforces a strict unidirectional data flow, ensuring that state changes are predictable and traceable, which makes debugging easier.
Middleware : Redux allows you to introduce middleware for side effects, such as handling asynchronous actions or logging.

41. Explain the concepts of actions, reducers, and the store in Redux.

Actions : Actions are plain JavaScript objects that describe an intention to change the state. They typically have a type field and can carry additional data. Actions are dispatched using the dispatch function and are the only way to trigger state changes in Redux.
Reducers : Reducers are pure functions that specify how the application's state should change in response to an action. They take the current state and an action as arguments and return a new state. Reducers are responsible for updating the state immutably.
Store : The store is a single, centralized object that holds the application's entire state tree. It has methods to dispatch actions, access the state, and subscribe to state changes. The store is created using the createStore function from the Redux library.

42. How can you connect a React component to the Redux store?

Import the necessary functions from react-redux connect.
Create a component that you want to connect.
Define mapStateToProps and/or mapDispatchToProps functions to specify which parts of the store's state and which actions the component needs access to.
Use the connect function to connect your component to the Redux store.

43. What is the purpose of the mapStateToProps and mapDispatchToProps functions?

mapStateToProps : This function is used to specify which parts of the Redux store's state should be mapped to props in your React component. It receives the current state as an argument and returns an object that defines the props you want to use in your component.
mapDispatchToProps : This function is used to map action creators to props in your React component. It allows you to dispatch actions in response to user interactions or other events. You can either directly pass action creators as an object or use bindActionCreators to bind action creators to the dispatch function.

44. What is the difference between Redux and the Context API for state management?

Complexity : Redux is generally better suited for complex state management scenarios with a large number of components that need access to shared state. The Context API is simpler and more lightweight, making it a good choice for smaller applications or simpler state management needs.
Predictability : Redux enforces a strict unidirectional data flow, which can make state changes more predictable and traceable. The Context API, while also unidirectional, allows for more flexible patterns, which might lead to less predictability in larger codebases.
Middleware : Redux provides a middleware system for handling side effects like asynchronous actions. The Context API does not offer built-in middleware, so you would need to handle side effects differently.
DevTools : Redux has powerful development tools like Redux DevTools that make debugging and inspecting state changes easier. The Context API does not have equivalent tools.
Community and Ecosystem : Redux has a well-established ecosystem with a large community and a wide range of extensions and middleware available. The Context API, being a part of React itself, is closely tied to React's ecosystem.

Component Styling

45. What are the different ways to style a React component?

CSS Files : You can use traditional CSS files and import them into your React components. This is the most common approach for styling React applications.
Inline Styles : React allows you to apply styles directly to JSX elements using inline style objects. This is done using the style attribute.
CSS Preprocessors : You can use CSS preprocessors like Sass or Less to write CSS with additional features and then import the processed CSS into your components.
CSS-in-JS : You can use libraries that allow you to write CSS directly in your JavaScript code, such as styled-components or Emotion.
CSS Modules : CSS Modules are a way to locally scope CSS styles to a specific component, preventing global CSS conflicts.
Third-Party UI Libraries : You can use third-party UI libraries like Material-UI or Ant Design, which provide pre-styled components for React applications.
Utility-First CSS : Libraries like Tailwind CSS provide utility classes that can be applied directly to your JSX elements.

46. Explain the advantages and disadvantages of CSS-in-JS solutions.

Scoped Styles : CSS-in-JS solutions typically generate scoped styles, reducing the risk of style conflicts in larger applications.
Dynamic Styles : You can create dynamic styles based on component props or application state, allowing for more responsive UIs.
Better Developer Experience : CSS-in-JS libraries often provide a better developer experience with features like automatic vendor prefixing, code completion, and type safety.
Component-Based Styling : Styles are tightly coupled with components, promoting component-based architecture.
Learning Curve : CSS-in-JS solutions can have a learning curve, especially for developers used to traditional CSS.
Performance Overhead : Some CSS-in-JS libraries may introduce a performance overhead due to dynamic style generation.
Tool Dependencies : You may need to rely on specific tooling and dependencies, which can add complexity to your project.
Bloat : Generated styles can sometimes be larger in size compared to hand-optimized CSS.
Limited Tooling : While there are several CSS-in-JS libraries available, the ecosystem may not be as extensive as traditional CSS tooling.

47. What is the CSS Modules approach, and how does it work in React?

File Structure : In your project, you create a CSS file for each component or module that needs styling. These files typically have the extension .module.css.
Import Styles : In your React component file, you import the CSS module using an import statement, which gives you an object containing the class names as properties.
Usage : You apply styles by using the class names from the imported module object as values for the className attribute in your JSX elements.

48. How can you use third-party UI libraries with React?

Install the Library : Use a package manager like npm or yarn to install the third-party UI library and its dependencies. For example, to install Material-UI:
Import Components : Import the components you need from the library into your React components. For example:
Use the Components : Use the imported components in your JSX code just like any other React component. Customize and configure them as needed based on the library's documentation and your application requirements.
Styling and Theming : Many third-party UI libraries provide customization options and theming support. Refer to the library's documentation to learn how to style and theme components according to your design preferences.
Advanced Configuration : Depending on the library, you might need to configure additional settings or use context providers for more advanced features. Again, consult the library's documentation for guidance.

Testing and Test Cases

49. What is the purpose of unit testing in React?

Detecting Bugs Early : Unit tests allow you to catch and fix bugs and regressions in your codebase early in the development process, reducing the chances of introducing issues as your project grows.
Ensuring Component Behavior : Unit tests help ensure that your React components behave as expected. You can verify that components render the correct UI, handle user interactions, and manage state appropriately.
Refactoring and Maintenance : Unit tests provide a safety net when refactoring or maintaining your code. They give you confidence that changes won't break existing functionality.
Documentation : Tests can serve as documentation for your code, showing how components are intended to work and providing examples of their usage.
Collaboration : Unit tests improve collaboration among team members by providing a clear definition of component behavior and expectations.
Continuous Integration (CI) and Deployment : Automated tests, including unit tests, are crucial in CI/CD pipelines to ensure that new code changes do not introduce regressions before deploying to production.

50. How can you write unit tests for React components?

Setup : Install the necessary testing libraries and configure your project for testing. For example, you can set up Jest and @testing-library/react:
Write Test Files : Create test files for your React components with a naming convention like Component.test.js. These files will contain your test cases.
Import Dependencies : Import the React component you want to test and any necessary dependencies, such as custom hooks or context providers.
Write Test Cases : Write test cases using testing utilities like render and fireEvent from @testing-library/react. Render your component and interact with it as a user would. Use assertions to check whether the component behaves as expected.
Run Tests : Execute your tests using a test runner like Jest. You can do this from the command line:
Analyze Test Results : Review the test results to ensure that all tests pass. If any tests fail, investigate and fix the issues in your code.
Refine and Maintain : As your codebase evolves, keep your tests up to date. Add new tests for new features or changes and refactor tests when necessary.

51. What is Jest, and how does it relate to React testing?

52. Explain the role of snapshots in Jest testing.

53. How do you perform end-to-end testing in React applications?

Server-Side Rendering (SSR)

54. What is server-side rendering (SSR) in React, and why is it important?

Improved SEO : Search engines can crawl and index the content easily because it's present in the initial HTML response.
Faster Initial Load : Users see content more quickly since the initial HTML includes rendered data, reducing the time spent waiting for JavaScript to load and execute.
Accessibility : Users with disabilities can start interacting with the page sooner, as the content is already available in the initial HTML.
Social Sharing : Social media platforms can scrape the initial HTML, ensuring that shared links display accurate content.
Performance : SSR can improve perceived performance, as users see something on the screen while JavaScript is still loading.

55. How can you implement SSR in a React application?

Server Setup : Set up a Node.js server or use a framework like Express.js to handle HTTP requests on the server-side.
Rendering on the Server : Use a library like ReactDOMServer (part of React) to render your React components on the server. You'll render the initial view to HTML and send it as part of the server response.
Route Handling : Implement route handling on the server to determine which component to render based on the URL requested by the client.
Data Fetching : Fetch any initial data needed for rendering from APIs or databases on the server.
Client Hydration : On the client-side, rehydrate the server-rendered HTML with JavaScript. React will attach event listeners and make the page fully interactive. This step is essential to enable client-side navigation and dynamic updates.
Webpack Configuration : Update your Webpack configuration to support both server-side and client-side rendering. Use tools like Webpack to bundle your server code and client code separately.
Routing : Implement routing logic that works on both the server and the client. Libraries like react-router can help achieve this.
Handling Data : Ensure that data fetched on the server is passed to the client correctly so that the client-side rendering can use the same data.
Error Handling : Implement error handling and 404 page rendering for both server and client-side routes.
Deployment : Deploy your SSR-enabled React application to a hosting platform or server that supports Node.js.

Performance Optimization

56. What are some common techniques for optimizing the performance of React applications?

Use Functional Components : Functional components have become the preferred way of defining components because they are typically more concise and performant than class components.
Minimize Re-renders : Avoid unnecessary re-renders by using shouldComponentUpdate or React.memo. This prevents components from rendering when their props or state haven't changed.
Use React DevTools : React DevTools provide insights into your component tree and help you identify performance bottlenecks.
Optimize Renders : Split your components into smaller ones to optimize rendering. Use PureComponent, React.memo, or shouldComponentUpdate to prevent unnecessary rendering.
Avoid Large Render Trees : Keep your component tree as shallow as possible. Deep component trees can lead to performance issues.
Lazy Loading and Code Splitting : Load only the code that's necessary for the current view to reduce initial bundle size.
Memoization : Cache expensive calculations or API calls to avoid redundant work.
Use PureComponent and React.memo : These help prevent re-renders when props or state haven't changed.
Virtualization : When dealing with large lists, use virtualization libraries like react-virtualized or react-window to render only the items currently visible in the viewport.

57. Explain lazy loading and code splitting in React.

Lazy Loading : Lazy loading involves loading parts of your application only when they are needed, reducing the initial load time. React allows lazy loading of components using React.lazy and dynamic imports.
Code Splitting : Code splitting is the process of breaking your code into smaller chunks to load only what's needed. You can achieve code splitting using tools like Webpack

58. What is memoization, and how can you implement it in React?

Webpack and Babel

59. What is Webpack, and how can you configure Webpack for a React project?

60. What is Babel, how do you configure Babel for a React project?

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20+ Critical React Interview Questions and Answers

With a component-based approach and one of the best possible rendering performances, React provides everything you need to create rich and engaging web apps. Companies like Facebook, Dropbox, and Instagram have long using it to solve user interface issues.

This open-source JavaScript library can be easily labeled as a bestseller in terms of web app development. According to statistics, React ranked second in the Developer Survey on Stack Overflow Top Web Frameworks (2019) .

Choosing React has many benefits, what about finding developers with great skills? As a product manager with little or no technical background, you may face challenges with conducting job interviews.

From interview advice to technical questions, this article contains everything you need to hire the best talents.

Things to Look When Hiring React Developer

Before diving into React interview questions and answers, it is necessary to clarify the essential hard and soft skills. As a matter of fact, React represents a library, so good professionals should also know coding languages and other instruments as well.

Your primary focus should be on checking candidates’ understanding of HTML, CSS, JSX, and JavaScript. It is then mandatory to check skills on ES6, Node, npm, Redux, Git, and more. No less important point is the general theory of computer science (you can check these fundamentals using software developer interview questions ).

Beyond technical skills, it is not superficial to determine the personal characteristics of a developer you are looking for. After all, you will communicate with a team a lot, so it is better to choose people who share the same vision or even interests. So, don’t forget to make a list of soft skills.

22 React Interview Questions to Clarify

When it comes to the technical side of a job interview, it may be challenging to determine candidates’ level of proficiency. To check the breadth and depth of knowledge, we suggest the following list of questions with comprehensive answers.

1. What is React, and how does it work?

A quite obvious yet no less important question. A popular front-end library, React focuses on building a UI of a single page web app (SAP) and mobile app. Firstly created by Facebook to handle the internal tasks of the company in 2011, it became open-source and available for everyone in 2013.

It is a server-side templating language with a set of callback-functions. More specifically, it is powered by JSX – a programming language that has a similar syntax to HTML.

The philosophy behind React is to separate UI (aka Presentation layer, or View) into components and work with them separately in real-time. Once these parts are created, you can easily modify a needed element without intervening in the whole structure.

So, it is an ideal tool for those who anticipate visually appealing user interfaces and need a stable platform to handle it.

2. What is the difference between library and framework?

React is often mentioned along with other frameworks like Angular, Node.js, or Vue.js. And although placing all of them together is quite logical, React relates to another specific category since it is a library.

Both frameworks and libraries represent a code written by other programmers to facilitate performing some tasks. The whole difference is that framework runs the code and controls all other environments, whereas the library is something developers use additionally (and control environment somewhere else).

To make this question more complex, ask a candidate to compare Angular and React. For instance, Angular is a full-fledged framework that includes a presentation layer and holds all the control over the program. React, in turn, is an additional instrument that doesn’t interfere with the flow of other processes of development.

3. Name advantages of using React over other libraries/frameworks

Although React is not a universal solution like other frameworks (Angular or Ember), it gives an impression of a tool that does one thing but does it well. You will not find a ‘default router’ or other pre-imposed DI dependencies there. Instead, you can reap a variety of other benefits. Here are some of the most important ones you should expect to hear from a candidate:

Component-based approach. Everything connected with the user interface can be treated as a separate component, or in other words, JavaScript function. This is salvation in many cases because you can divide everything into classes, modules, or other elements and conduct more comfortable development. A component is a kind of building block that forms the entire user interface. With just a snippet of source code, developers can recognize how each component will eventually look like. You can also perform various manipulations with components like reusing, separating, or arranging them as you want, etc. This approach is especially useful for large-scale projects where there is a need to share code with other developers. Such structure of a web page may look like this:

open-source JavaScript library used to manage application Statean architecture and not a framework or library has multiple storeshas single store yesno noyes immutablemutable

18. How code base differs for iOS and Android devices in React Native?

A simple yer tricky question at the same time. As a matter of fact, there are no differences since React Native provides an advantage to use the same code base for all platforms.

19. Give an example of how to use synthetic events

This type of events is created opposite to real events. Their main purpose is to normalize events and make sure they have the same properties across different browsers and platforms.

20. What is meant by the process of reconciliation?

It is a widespread process in React, so it is necessary to clarify whether your candidate knows how it works. It is primarily linked to updating data and making changes more predictable.

If to delve into more details, reconciliation reproduces the following procedure: once a component’s props or state undergoes changes, React compares it with the previous alteration. If these two changes are different, we get reconciliation.

21. Describe the purpose of React Fiber

At its core, React offers several ways to store a state. Basically, it is a tree of internal elements like DOM nodes. But since these elements did not always cope excellently with increased performance tasks (like animation), starting from version 16, React presented a more advanced algorithm for this purpose called Fiber.

Its main advantage is the ability to divide render performance into units and distribute them across multiple frames. Using Fiber, developers can pause or cancel the playback of DOM tree updates at any time. It is also possible to configure the prioritization of different updates.

22. How to conduct re-rendering of a component?

This may be a final point you can touch on while preparing the list of React.js interview questions and answers. The most comfortable way to invoke rerendering is to change the state and props. Moreover, it is possible to use methods like this.setState and this.forceUpdate .

Project Success Starts With A Team

Every specialist in your team can be viewed as a puzzle piece in a much larger canvas. Your final result will mainly depend on how your team collaborates and whether everyone shares a common goal. That’s why it is so crucial to approach the task of hiring and forming a team correctly. Be sure you have picked the person who will add new experiences and bring inspiration to your company culture.

Struggling to build a team? Hire React developers with Bridge. Since we have access to a tremendous pool of talents worldwide, we can quickly find the best fit for your project’s requirements and budget.

We are more than global recruitment because we are focused on organizing a perfect team for your idea. That’s why we manage teams and care about your workers’ comfort so that they can be a valuable asset to your mission.

If you are curious about the pricing, run numbers of needed specialists’ salary using our smart calculator .

Have something to ask? Don’t hesitate to contact us .

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Traditional Tech Talent Acquisition Methods Are Dead: Optimize Your Approach With New Practices

The demand for tech talent continuously shifts, influenced by a variety of factors like the ever-changing technology, economic tides, and, of course, unexpected twists like the pandemic. Just think of it ‒ not long ago, the COVID-19 pandemic accelerated the digital transformation across industries, intensifying the need for skilled technical professionals. And now? Tech giants …

Traditional Tech Talent Acquisition Methods Are Dead: Optimize Your Approach With New Practices Read More »

AI Vetting Platforms for Hiring Software Engineers: Choosing the Right Tools for Your Needs

Finding and recognizing qualified software engineers has never been easy. Recruiters and hiring managers often spend a lot of time manually reviewing resumes and assessing technical skills in pursuit of a perfect candidate. Traditional evaluation methods often lead to human error and unconscious biases while making the process tedious and time-consuming. Being more than a …

AI Vetting Platforms for Hiring Software Engineers: Choosing the Right Tools for Your Needs Read More »

Choosing the Right Programming Language for Your Machine Learning Project

When picking the best programming language for an ML project, the first thing you need to understand is that there’s no one-size-fits-all manual or a neat step-by-step guide. As cliché as it may sound, the choice largely depends on your project’s specific requirements. At the same time, what you can and should do is research …

Choosing the Right Programming Language for Your Machine Learning Project Read More »

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Reactjs tutorial: a step-by-step guide to learn react, the best guide to know what is react, a beginners guide to react props, reactjs state: setstate, props and state explained, reactjs components: type, nesting, and lifecycle, react with redux tutorial: learn the basics, flutter vs. react native : which one to choose in 2024, top 40 reactjs interview questions and answers for 2024, the best guide to understanding react and express, introduction to typescript with react: the most informative guide, all you need to know about how to create a facebook clone using react, how to create a youtube clone using react, all you need to know about react 18: new features and improvements, 40 reactjs advanced interview questions: get hired in 2024.

Lesson 7 of 12 By Taha Sufiyan

React is quite the buzzword in the industry these days. As of now, React is the most popular front-end technology that more and more companies are using, and if you are preparing for a job interview , this is ReactJS interview questions tutorial is just the right one for you. Here's a comprehensive list of all the common ReactJS interview questions from basic to advanced levels that are frequently asked in interviews.

Most Asked ReactJS Interview Questions

What is ReactJS?
Why ReactJS is Used?
How Does ReactJS work?
What are the features of ReactJS?
What is JSX?
How to create components in ReactJS?
What are the advantages of ReactJS?
Differentiate between real DOM and virtual DOM?
What are forms in ReactJS?
How is React different from React Native?

Basic Level - ReactJS Interview Questions

Here are some React Interview Questions on basic concepts.

1. What are the features of React?

	JSX is a syntax extension to JavaScript. It is used with React to describe what the user interface should look like. By using JSX, we can write structures in the same file that contains code.
	are the building blocks of any React application, and a single app usually consists of multiple components. It splits the user interface into independent, reusable parts that can be processed separately.
	React keeps a lightweight representation of the real DOM in the memory, and that is known as the virtual DOM. When the state of an object changes, virtual DOM changes only that object in the real DOM, rather than updating all the objects.
	React’s one-way keeps everything modular and fast. A unidirectional data flow means that when designing a React app, you often nest child components within parent components.
	React updates only those components that have changed, rather than updating all the components at once. This results in much faster web applications.

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2. What is JSX?

JSX is a syntax extension of JavaScript. It is used with React to describe what the user interface should look like. By using JSX, we can write HTML structures in the same file that contains JavaScript code.

3. Can web browsers read JSX directly?

Web browsers cannot read JSX directly. This is because they are built to only read regular JS objects and JSX is not a regular JavaScript object
For a web browser to read a JSX file, the file needs to be transformed into a regular JavaScript object. For this, we use Babel

4. What is the virtual DOM?

DOM stands for Document Object Model. The DOM represents an HTML document with a logical tree structure. Each branch of the tree ends in a node, and each node contains objects.

React keeps a lightweight representation of the real DOM in the memory, and that is known as the virtual DOM. When the state of an object changes, the virtual DOM changes only that object in the real DOM, rather than updating all the objects. The following are some of the most frequently asked react interview questions.

5. Why use React instead of other frameworks, like Angular?

	React makes it easier to create dynamic web applications because it provides less coding and provides more functionality, whereas, with JavaScript applications, code tends to get complex very quickly.
	React uses virtual DOM, which makes web applications perform faster. Virtual DOM compares its previous state and updates only those components in the real DOM, whose states have changed, rather than updating all the components — like conventional web applications.
	Components are the building blocks of any React application, and a single app usually consists of multiple components. These components have their own logic and controls, and they can be reused through the application, which, in turn, dramatically reduces the development time of an application.
	React follows a unidirectional data flow. This means that when designing a React app, we often nest child components within parent components. And since the data flows in a single direction, it becomes easier to debug errors and know where the problem occurs in an application at the moment.
	Facebook has released a chrome extension that we can use to debug React applications. This makes the process of debugging React to web applications faster and easier.

6. What is the difference between the ES6 and ES5 standards?

This is one of the most frequently asked react interview questions.

These are the few instances where ES6 syntax has changed from ES5 syntax:

Components and Function

exports vs export

require vs import

7. How do you create a React app?

These are the steps for creating a React app:

Install NodeJS on the computer because we need npm to install the React library. Npm is the node package manager that contains many JavaScript libraries, including React.

Install the create-react-app package using the command prompt or terminal.

Install a text editor of your choice, like VS Code or Sublime Text.

8. What is an event in React?

An event is an action that a user or system may trigger, such as pressing a key, a mouse click, etc.

React events are named using camelCase, rather than lowercase in HTML.
With JSX, you pass a function as the event handler, rather than a string in HTML.

9. How do you create an event in React?

A React event can be created by doing the following:

10. What are synthetic events in React?

Synthetic events combine the response of different browser's native events into one API, ensuring that the events are consistent across different browsers.
The application is consistent regardless of the browser it is running in. Here, preventDefault is a synthetic event.

11. Explain how lists work in React

We create lists in React as we do in regular JavaScript. Lists display data in an ordered format
The traversal of lists is done using the map() function

12. Why is there a need for using keys in Lists?

Keys are very important in lists for the following reasons:

A key is a unique identifier and it is used to identify which items have changed, been updated or deleted from the lists
It also helps to determine which components need to be re-rendered instead of re-rendering all the components every time. Therefore, it increases performance, as only the updated components are re-rendered

13. What are forms in React?

React employs forms to enable users to interact with web applications.

Using forms, users can interact with the application and enter the required information whenever needed. Form contain certain elements, such as text fields, buttons, checkboxes, radio buttons, etc
Forms are used for many different tasks such as user authentication, searching, filtering, indexing, etc

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14. How do you create forms in React?

We create forms in React by doing the following:

The above code will yield an input field with the label Name and a submit button. It will also alert the user when the submit button is pressed.

15. How do you write comments in React?

There are basically two ways in which we can write comments:

Single-line comments

Multi-line comments

16. What is an arrow function and how is it used in React?

An arrow function is a short way of writing a function to React.
It is unnecessary to bind ‘this’ inside the constructor when using an arrow function. This prevents bugs caused by the use of ‘this’ in React callbacks.

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17. How is React different from React Native?


Release	2013	2015
Platform	Web	Mobile – Android, iOS
HTML	Yes	No
CSS	Yes	No
Prerequisites	JavaScript, HTML, CSS	React.js

18. How is React different from Angular?


Author	Google	Facebook
Architecture	Complete MVC	View layer of MVC
DOM	Real DOM	Virtual DOM
Data-Binding	Bi-directional	Uni-directional
Rendering	Client-Side	Server-Side
Performance	Comparatively slow	Faster due to Virtual DOM

In case you have any doubts about these Basic React interview questions and answers, please leave your questions in the comment section below.

ReactJS Interview Questions on Components

Here are some React Interview Questions on components.

19. What are the components in React?

Components are the building blocks of any React application, and a single app usually consists of multiple components. A component is essentially a piece of the user interface. It splits the user interface into independent, reusable parts that can be processed separately.

There are two types of components in React:

Functional Components: These types of components have no state of their own and only contain render methods, and therefore are also called stateless components . They may derive data from other components as props (properties).

function Greeting(props) {

return <h1>Welcome to {props.name}</h1>;

}

Class Components: These types of components can hold and manage their own state and have a separate render method to return JSX on the screen. They are also called Stateful components as they can have a state.

class Greeting extends React.Component {

render() {

return <h1>Welcome to {this.props.name}</h1>;

}

20. What is the use of render() in React?

It is required for each component to have a render() function. This function returns the HTML, which is to be displayed in the component.
If you need to render more than one element, all of the elements must be inside one parent tag like <div>, <form>.

21. What is a state in React?

The state is a built-in React object that is used to contain data or information about the component. The state in a component can change over time, and whenever it changes, the component re-renders.
The change in state can happen as a response to user action or system-generated events. It determines the behavior of the component and how it will render.

22. How do you implement state in React?

23. How do you update the state of a component?

We can update the state of a component by using the built-in ‘setState()’ method:

24. What are props in React?

Props are short for Properties. It is a React built-in object that stores the value of attributes of a tag and works similarly to HTML attributes.
Props provide a way to pass data from one component to another component. Props are passed to the component in the same way as arguments are passed in a function.

25. How do you pass props between components?

26. What are the differences between state and props?


Use	Holds information about the components	Allows to pass data from one component to other components as an argument
Mutability	Is mutable	Are immutable
Read-Only	Can be changed	Are read-only
Child components	Child components cannot access	Child component can access
Stateless components	Cannot have state	Can have props

27. What is a higher-order component in React?

A higher-order component acts as a container for other components. This helps to keep components simple and enables re-usability. They are generally used when multiple components have to use a common logic.

28. How can you embed two or more components into one?

We can embed two or more components into one using this method:

29. What are the differences between class and functional components?


State	Can hold or manage state	Cannot hold or manage state
Simplicity	Complex as compared to the stateless component	Simple and easy to understand
Lifecycle methods	Can work with all lifecycle methods	Does not work with any lifecycle method
Reusability	Can be reused	Cannot be reused

Class components example:

Functional components example:

30. Explain the lifecycle methods of components.

getInitialState(): This is executed before the creation of the component.
componentDidMount(): Is executed when the component gets rendered and placed on the DOM.
shouldComponentUpdate(): Is invoked when a component determines changes to the DOM and returns a “true” or “false” value based on certain conditions.
componentDidUpdate(): Is invoked immediately after rendering takes place.
componentWillUnmount(): Is invoked immediately before a component is destroyed and unmounted permanently.

So far, if you have any doubts about the above React interview questions and answers, please ask your questions in the section below.

ReactJS Redux Interview Questions

Here are some ReactJS Interview Questions on the ReactJS Redux concept.

31. What is Redux?

Redux is an open-source, JavaScript library used to manage the application state. React uses Redux to build the user interface. It is a predictable state container for JavaScript applications and is used for the entire application’s state management.

32. What are the components of Redux?

Store: Holds the state of the application.
Action: The source information for the store.
Reducer: Specifies how the application's state changes in response to actions sent to the store.

33. What is the Flux?

Flux is the application architecture that Facebook uses for building web applications. It is a method of handling complex data inside a client-side application and manages how data flows in a React application.

There is a single source of data (the store) and triggering certain actions is the only way way to update them.The actions call the dispatcher, and then the store is triggered and updated with their own data accordingly.

When a dispatch has been triggered, and the store updates, it will emit a change event that the views can rerender accordingly.

34. How is Redux different from Flux?


1.	Redux is an open-source JavaScript library used to manage application State	Flux is an architecture and not a framework or library
2.	Store’s state is immutable	Store’s state is mutable
3.	Can only have a single-store	Can have multiple stores
4.	Uses the concept of reducer	Uses the concept of the dispatcher

So far, if you have any doubts about these React interview questions and answers, please leave your questions in the section below.

ReactJS Router Questions

Here are some ReactJS Interview Questions on React Router concepts.

35. What is React Router?

React Router is a routing library built on top of React, which is used to create routes in a React application. This is one of the most frequently asked react interview questions.

36. Why do we need to React Router?

It maintains consistent structure and behavior and is used to develop single-page web applications.
Enables multiple views in a single application by defining multiple routes in the React application.

37. How is React routing different from conventional routing?


1.	Single HTML page	Each view is a new HTML file
2.	The user navigates multiple views in the same file	The user navigates multiple files for each view
3.	The page does not refresh since it is a single file	The page refreshes every time user navigates
4.	Improved performance	Slower performance

38. How do you implement React routing?

We can implement routing in our React application using this method:

Considering we have the components App , About , and Contact in our application:

Hope you have no doubts about this ReactJS interview questions article, in case of any difficulty, please leave your problems in the section below.

ReactJS Styling Questions

Here are some ReactJS Interview Questions on Styling concept ReactJS.

39. How do you style React components?

There are several ways in which we can style React components:

Inline Styling

JavaScript Object

CSS Stylesheet

40. Explain the use of CSS modules in React.

The CSS module file is created with the .module.css extension
The CSS inside a module file is available only for the component that imported it, so there are no naming conflicts while styling the components.

These are all the basic to advanced ReactJS interview questions that are frequently asked in interviews. We hope these ReactJS interview questions will be helpful in clearing your interview round. All the best for your upcoming job interview! Suppose you want to learn more about ReactJS components, I suggest you click here!

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Hiring Interview Tips

ReactJS Interview Questions and Answers for 2024

By Aran Davies

8 years of experience

Aran Davies is a full-stack software development engineer and tech writer with experience in Web and Mobile technologies. He is a tech nomad and has seen it all.

Whether you’re an entrepreneur, a project manager in a large enterprise, or a CTO, you are well aware that the success of your project rests on your ability to find top developers.

In this guide, you‘ll find example ReactJS interview questions and answers you can refer to when seeking a new ReactJS developer to make your dynamic user interfaces come to life. You‘ll also get some practical advice on how to use these questions to reliably identify the best developers for the job.

First Things First: Select Your Job Requirements

ReactJS is a flexible JavaScript library that can be used in different kinds of applications and alongside other web technologies. To find a ReactJS that’s a great fit for your project, you need to get clear on what your job requirements are.

Think about the type of application you are developing, and what technical skills are critical to its success. You also need to think of the softer skills that a candidate needs to work well with the rest of your team.

Some example ReactJS job requirements include:

Essential web development skills – E.g. HTML, Javascript
ReactJS-specific skills – E.g. JSX
Library/toolkit experience – E.g. Redux/MobX
Design skills – Building scalable applications, using the Flux architecture
Communication skills – Discussing problems and constraints with your clients
Being a self-starter – If you don‘t want to do any hand-holding

Avoid making a laundry list of non-essential skills for your perfect ReactJS developer. Instead, focus on what your candidate will actually be doing day-to-day. Keep your requirements list as short as possible. Cut anything they can do without or learn on the job.

With clearly stated requirements, you‘ll be able to choose the right ReactJS coding interview questions and have a much better idea of what kinds of responses you are looking for.

In this guide, we categorize the questions based on the level of expertise you‘re looking for. Junior Developer, Mid-level Developer, and Senior Developer. If you‘re unsure exactly which one you need, take a look at the different sections for a summary of what you can expect from each.

Junior Developer interview questions
Mid-level Developer interview questions
Senior Developer interview questions

ReactJS Junior Interview Questions

Junior developers have the least experience and demand the lowest salary. It‘s important to note before hiring a junior that they need to work under the guidance of more experienced developers.

Skill Requirements for Junior ReactJS Developers

Basic HTML, CSS, and Javascript skills
Foundational ReactJS knowledge
Learning on the job
Following instructions and receiving feedback
Thinking like a programmer

Example Basic React Interview Questions and Answers

Note: Important keywords are underlined in the answers. Bonus points if the candidate mentions them!

Or save yourself time and request a team to match your needs right away.

Question 1: Write an example of a simple HTML document with some header information and some page content. Requirement: Basic HTML skills

Answer: HTML documents are different, but they follow a basic structure of head and body. The different sections are marked with tags such as DOCTYPE, html, head, title, meta, body, h1, p.

For example:

Question 2: What is the purpose of the following code? Requirement: Using CSS with React

Answer: This code adds a CSS class to a component by passing a string as the className prop. Here the class depends on the component props or state which is common with ReactJS.

Question 3: Explain what a Javascript callback function is and provide a simple example. Requirement: Basic Javascript skills

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Answer: A callback function is a function that is called after another function has finished executing. A callback function is passed to another function as an argument and is invoked after some operation has been completed. For example:

Question 4: Briefly describe ReactJS in one or two sentences. Requirement: Foundational ReactJS knowledge

Answer: ReactJS is an open-source Javascript library . It is designed for building dynamic single-page applications with multiple components that require less coding than doing everything yourself in Javascript. Sometimes it is called a framework, but that‘s a question of terminology.

Question 5: What is ReactJS? How does it compare to other JavaScript frameworks? Requirement: Foundational ReactJS knowledge

Answer: Here you are getting a feel for the candidate‘s knowledge, ideas, and opinions of what React is, and how it fits into the JavaScript ecosystem. Some points to look out for:

React is an open-source JavaScript library maintained by Facebook;
It‘s designed for building dynamic and interactive UIs (user interfaces) for web and mobile applications;
It is used to help you build reusable UI components;
Sometimes seen as the ’View‘ in the ’Model-View-Controller‘ (MVC) architecture.;
This means it is a relatively small framework (compared to, say, Angular, which takes care of the whole front end) and can be used easily with other libraries and integrated into many applications;
React isn‘t as opinionated about an application‘s architecture as other frameworks like Angular and Vue.js;
Very strong community with many supporting tools and packages.

Question 6: How would you learn about a new Javascript library? Requirement: Learning on the job

Answer: Web development is changing all the time, and developers need to be able to learn constantly. Here you are finding out how the candidate approaches the learning process. You want to see that they have an idea of what information they will need and where to find it. For Javascript libraries, that means looking up online tutorials and digging into the documentation.

Question 7: Describe a time you received feedback on a programming task. How did it help you become a better programmer? Requirement: Following instructions and receiving feedback

Answer: This is a typical open-ended question. The candidate should demonstrate they can accept, understand and act on feedback.

Question 8: A common type of problem we have to solve at this company is _____. How might you think about finding a solution? Requirement: Thinking like a programmer

Answer: In this question, the problem should be directly related to the work the candidate will actually be doing day-to-day for you. You aren‘t looking for a perfect answer or even necessarily a correct answer. Instead, listen to how they approach solving a problem, their ability to break a problem down into parts, and if they can anticipate problems.

ReactJS Mid-level Developer Interview Questions

Mid-level developers are the workhorses of the software development world. They are fairly experienced and you can rely on them to execute routine tasks with skill and efficiency. They can also mentor junior developers.

However, if you need a developer with high-level design skills to lead your project, you might want to skip down to the expert-level ReactJS programming interview questions.

Note: Important keywords are underlined in the answers. Look out for them in interviews!

Question 9: List some advantages and limitations of ReactJS. Requirement: Expert ReactJS knowledge

Advantages:

Free and Open Source;
Highly adaptable;
Virtual DOM, a lightweight javascript object, makes it extremely efficient;
JSX increases code reliability and makes mixing HTML and JavaScript much easier;
Makes UI testing easier;
Developer tools such as the React.js chrome extension for debugging;
More SEO friendly than some other JavaScript frameworks;
Large supporting community.

Limitations:

Library, not a framework. It handles the UI only. You need to use other libraries for other parts of your application;
No set way to structure applications, meaning you have to figure it out for yourself;
Its flexibility can easily let developers make poor choices;
JSX and inline templating can make the code quite complex, especially for novice developers;
A different way of thinking and learning curve compared to other frameworks like Angular.

Question 10: Explain what the Virtual DOM is in ReactJS and why it is necessary. Requirement: Expert ReactJS knowledge

Answer: Virtual DOM is one of the key concepts in React. A good candidate should be able to explain the problems with DOM manipulation and why the virtual DOM helps.

DOM (Document Object Model) manipulation is how web applications update the HTML on a webpage to make it dynamic and interactive. However, updating the DOM is slow and most JavaScript frameworks update it more than they need to. Usually, by updating the entire DOM every time a small change is made on a page.

React tries to reduce this waste by only updating the parts of the DOM that are actually updated. To do this, React keeps track of a corresponding lightweight ’Virtual DOM Object‘ that can be updated and checked for changes much faster than the real DOM.

React uses Virtual DOM snapshots to work out exactly which parts of a page need updating, and updates only those in the real DOM rather than creating a new DOM. By using a virtual DOM, web applications can be made much faster and more efficient.

Question 11: What is JSX? How does it relate to ReactJS? Give a quick example. Requirement: Expert ReactJS knowledge

Answer: JSX (JavaScript XML) is a preprocessor that allows you to include XML syntax in your JavaScript . Its basic function is to make code more intuitive and easier to read. The React library realizes that often JavaScript and HTML are strongly related, and having them in separate HTML and JavaScript files makes things confusing. Here are some examples of JSX code:

It isn‘t mandatory to use JSX with React, but it makes a lot of sense to do so.

Question 12: What are the Components in React? Requirement: Expert ReactJS knowledge

Answer: ReactJS follows a component-based approach . That means that components make up the building blocks of a ReactJS application by splitting the UI up into many separate, reusable components . Components can be thought of like JavaScript functions. They accept inputs called props and return React elements that describe what should be presented to the user.

Question 13: What do the following two ReactJS code segments do? Requirement: Expert ReactJS knowledge

Answer: Both of the above define components in React. The first is a functional component and the second uses an ES6 class to define a component. However, from the perspective of ReactJS, the above two components are exactly equivalent .

Question 14: What does render() do? Requirement: Expert ReactJS knowledge

Answer: Every React component has a render() function. Render() returns exactly one React element that represents a native DOM component. Multiple HTML elements must be grouped inside one tag, for example, <group>.

Question 15: What‘s your experience with the Flux architecture pattern? Requirement: Experience using Flux architecture

Answer: Many ReactJS applications use Flux architecture rather than MVC. Its key feature is that it enforces unidirectional data flow . The three major parts of a flux application are the:

View (React components)

Question 16: What‘s your experience with the Redux library? Requirement: Experience using libraries you use

Answer: Redux is a state management library commonly used with React. You‘re checking if your candidate can talk about using Redux (or another library you use) to build testable applications that can run across different development environments with predictable behavior.

Single source of truth;
The state is read-only;
Changes are made with Pure functions.

Question 17: What are some testing tools you would use for unit testing in a ReactJS application? Requirement: Unit testing UI components

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Answer: Unit testing is a technique to test whether isolated segments of code are functioning properly. Some tools for testing ReactJS applications include Enzyme, Jest, react-testing-library, React, unit, Skin-deep, and Unexpected-react.

Question 18: What is the Children prop? Requirement: Understanding JSX and composition.

Answer: A special prop that takes data between tags. For example:

props.children in the component Header will be equal to “Hello world”.

Question 19: Can you describe the React Component lifecycle? When you should make an asynchronous call? Requirement: React component’s lifecycle. React 16/17 breaking changes.

The usual component lifecycle looks like this:

constructor
componentWillMount (should be mentioned, that in React 16/17 this hook is deprecated and replaced by getDerivedStateFromProps)
componentDidMount
componentWillUpdate (in React 16/17 it was replaced by getDerivedStateFromProps)
shouldComponentUpdate
getSnapshotBeforeUpdate
componentDidUpdate
componentWillUnmount

Before React 16 it was ok to make a call in componentWillMount. But now that componentWillMount has become deprecated the earliest hook for the asynchronous calls is componentDidMount. It‘s also ok to make calls from ’componentDidUpdate‘.

Example ReactJS Expert Interview Questions

Now for the most difficult ReactJS interview questions and answers for experienced ReactJS developers. These are for finding the developers that can design and build you a world-class React application.

Skill Requirements for Senior ReactJS Developers

Expert ReactJS knowledge;
Flux architecture;
Designing for specific requirements (e.g. security, scalability);
Asynchronous programming;
Maintaining and upgrading applications;
Experience in other JavaScript frameworks and libraries (e.g. Node JS);
Efficient programming and clean code;
End-to-end and unit testing;
Leadership skills;
Clear communication skills;
Mentoring less experienced developers.

Question 20: What React patterns do you know? Can you describe what is HOC or Render prop? Requirements: React patterns and app architecture

Answer: There are a lot of patterns for React, such as:

Higher Order Component (HOC);
Render Prop;
Function as children;
Container and Presentational components.

HOC is the acronym of High Order Component, which is very similar to High Order Function. It‘s a function that takes a component and returns a new component with some changes. For example, Redux connect functions is the HOC, which takes a component and returns a component with Redux State and Actions.

Render Prop is the pattern for creating components with “render callback”. Usually, it is used for the same purpose as HOC. For example, it is used by React-Router 4 and Context API.

Question 21: Do you have any experience with React 16 Context API? What problems does it solve? Requirements: React Context API

Answer: Context API provides a way to pass data through the component tree without having to pass props down manually at every level. It can be used to share global data (such as user, theme, or any other information) between child components.

Question 22: What is React Portal and React Fragment? What problems do they solve? Requirements: React Portal and React Fragment.

Answer: Portals provide a first-class way to render a child component into a DOM node that exists outside the DOM hierarchy of the parent component. For example, it can simplify Modal window rendering or changing information inside <head></head> tags.

React Fragments are for grouping a list of children without adding extra nodes to the DOM. Because React forces us to return only one object from render, we should wrap a child component into a div or a similar container tag. Sometimes it‘s simpler to return several tags without a container to implement some features.

Question 23: What is React Reconciliation? How do you avoid performance issues? Requirements: React rendering algorithm

Answer: Reconciliation is the process that React uses to efficiently update the DOM. It does this by only updating the parts that need it. At a single point in time, the render() function will create a tree of React elements. On the next state or props update, that render() function will return a different tree of React elements. React then needs to figure out how to efficiently update the UI to match the most recent tree. The process of figuring this out is reconciliation.

To avoid performance issues we should:

Add ’key‘ prop;
Use PureComponent with React.memo();
Memoize functions (reselect, for example).

Question 24: What is the ’PropTypes‘ library? Requirements: PropType library, data validation

Answer: PropTypes is the type-checking addition to React Library, which exports a range of validators to make sure the data component receive is valid. Using it is a good idea because it reduces the number of bugs and makes components self-documented.

Question 25: Do you have any experience in code splitting with React? Can you describe how a bundle can be split into smaller chunks? Requirements: Webpack bundling, React.lazy or React Loadable

Answer: React has tools to avoid large bundle sizes. You can use React Loadable, because of its simplicity, but React 16.6 also has solutions such as React Lazy.

React Loadable is a small library for wrapping components into bundles. It provides nice features such as placeholders for “Loading” and “Error” components and flash delay. When a component is wrapped, Webpack will move it into a new bundle.

Open-Ended Questions

Once you‘ve established that your developer is an expert in ReactJS, you should continue the interview by asking some messy and open-ended questions to spark a discussion. Tailor these questions to fit your own job requirements and don‘t be afraid to ask follow-up questions.

Question 26: If you could use whatever tools you like to build our ____ application, what would you use? Requirement: Design skills, understanding of requirements

Answer: In this question, you are getting a feel for the type of developer you are talking to and how they like to code. You are especially looking for developers that try to understand the requirements first. It‘s a big red flag if a developer gives a list of libraries and tools without understanding the task.

Question 27: How are you involved in the ReactJS community? Requirement: Passion for web development

Answer: This is a popular question for coding interviews because community and open source involvements are clear indicators of a passionate developer.

Question 28: Describe a time you fixed an error in a web application. How did you approach the problem? What debugging tools did you use? What did you learn from this experience? Requirement: Debugging, Breaking down a problem into parts

Debugging is one of the key skills for any web developer. However, the real skill is in breaking the problem down practically rather than finding small errors in code snippets. Debugging often takes hours or even days, so you don‘t have time in an interview setting. Asking these questions will give you an idea of how your candidate approaches errors and bugs.

Answer: In the candidate‘s response you should look out for things like:

A measured, scientific approach;
Breaking down the problem into parts;
Finding out how to reproduce the error;
Expressing and then testing assumptions;
Looking at stack traces;
Getting someone else to help/take a look;
Searching the internet for others that have had the same problem;
Writing tests to check if the bug returns;
Checking the rest of the code for similar errors;
Turn problems into learning experiences.

Question 29: What’s the most important thing to look for or check when reviewing another team member’s code? Requirement: Mentoring less experienced developers, Leadership skills

Answer: Here you‘re checking for analysis skills, knowledge of mistakes that less experienced developers make, keeping in mind the larger project, and attention to detail.

A good answer might mention code functionality, readability and style conventions, security flaws that could lead to system vulnerabilities, simplicity, regulatory requirements, or resource optimization.

Question 30: What tools & practices do you consider necessary for the Continuous Integration and Delivery of a web application using ReactJS? Requirement: DevOps systems design, Maintain, and upgrade applications

Example Answer: One example of React applications is create-react-app. It provides a lot of features for building. It is also good to separate config files for development and production.

As for deployment, it depends on what hosting we are using. For example, for Firebase there is nothing complicated: usually, we run the build script and run the firebase deployment script. Same for GitHub pages, Heroku, and Netlify.

For deployment to basic hosting with client-side rendering they might mention nginx and node server.

Question 31: What is your favorite method to fill a React Component with CSS? Do you have an experience with CSS-In-JS libraries? Requirement: React component styling

Answer: There are several options to style components:

by className;
by inline styles;
third-party dom libraries for styling components such as StyledComponents, Radium, CssModules, etc.

Question 32: What is the purpose of React Refs? Can you describe a problem you can solve by using it? Requirement: React DOM interactions.

Answer: Refs is the method to access a DOM node or React component inside a parent component. A common problem it solves is to focus input or get the child component‘s fields. It‘s better to use refs as little as possible.

Important knowledge areas of ReactJS and software development where you should focus on

We talked about the skillsets and ReactJS interview questions. While these give you a guideline about the specifics that you should look for during the interview, there are general aspects to consider too. These carry plenty of importance since you want effective developers.

We recommend that you look for the following general skills of ReactJS:

1. The knowledge of ReactJS lifecycle methods

ReactJS components have their “lifecycle methods”. You can override them to run your code at particular times during the processing of your application. These lifecycle methods belong to various categories, e.g.:

Unmounting;
Error-handling.

Common examples of lifecycles are the following:

“ComponentWillUpdate”;
“ComponentDidUpdate”.

Note that ReactJS developers should understand how the re-rendering a component happens. With ReactJS, the default is to re-render every time the state changes.

2. The knowledge of using the “PureComponent” method

ReactJS developers will need to prevent rendering in some cases. The application might receive new props or a new state. Pure components being the fastest components can replace any component with only a render() function. A developer might think of using the “ShouldComponentUpdate” method in such cases, however, this needs careful consideration.

The “ShouldComponentUpdate” method helps more with performance optimization. If a ReactJS developer uses it to prevent re-rendering, then this might lead to bugs in the program.

ReactJS developers should use the “PureComponent” method for this purpose. It performs a shallow comparison of props and the state. This reduces the likelihood of skipping a required update, which is safer.

3. The knowledge of “setState”

ReactJS developers should know how to use “setState”. It provides an important capability since it schedules updates of the “state” object of a component.

Developers should know that calls to “setState” are asynchronous. This means that ReactJS developers can’t expect the app to reflect the new value immediately after calling “setState”. They need to use an “updater” function too.

ReactJS developers should remember that “setState” calls are in batches. They should schedule updates in a chain by using this property, which prevents conflicts between updates.

4. The knowledge of ReactJS “class components”

ReactJS developers need to have a thorough knowledge of functional and class components. Being a JavaScript framework, ReactJS conforms to JavaScript standards and practices greatly. Programmers need to write a JavaScript function to define a class component.

only class components were used for state management and lifecycle methods before React hooks were introduced. Components in ReactJS help developers to manage the UI effectively. They can divide the UI into independent parts, moreover, they can make these parts reusable.

ReactJS provides detailed API reference documentation. Developers should be familiar with it, which will help them to work with class and react function components. They should know how to use a JavaScript object.

5. Familiarity with “controlled components”

ReactJS developers should know the concept of “controlled components” very well. These are specific types of input form elements. In the case of these elements, a ReactJS component would render the form. It also controls the events in the form upon receiving the next user input.

The React state will drive the value of the input. ReactJS developers should know that they need to write more lines of code to use a “controlled component”. They should know the advantages though, i.e., passing the values to other UI elements.

6. The knowledge of using “stateless components”

ReactJS programmers should know the concept of “stateless components”. JavaScript functions play a part here since a stateless component is a functional component. Such functional components take props as inputs, subsequently, they return a ReactJS element.

“Stateless components” don’t have a lifecycle. ReactJS calls the “MyStatelessComponent” function to render a “stateless component”, and developers should know about it.

7. The knowledge of using “Higher-Order Components” in ReactJS

ReactJS developers should know how to reuse component logic effectively. This makes their work easier, and “Higher-Order Components” (HOCs) can help. These are advanced techniques available to ReactJS programmers.

HOCs are not part of the ReactJS APIs but are patterns derived from react’s compositional nature. They are functions to take one component and return a new component.

8. The knowledge of ReactJS “stateful components”

We talked about “stateless components”, however, ReactJS also have “stateful components”. Such a component contains the state object. States allow the creation of dynamic and interactive components. ReactJS developers should know how to use these components. They also need to know how to initialize “this.state”.

9. The knowledge of using ReactJS “arrow functions”

ReactJS developers should know how to pass functions to components. In this context, knowing an “arrow function” becomes important. These functions present easy ways to pass parameters to “callback” functions.

The knowledge of passing functions has other benefits too. E.g., developers can pass an event handler like “onClick” to a component.

10. The knowledge of “React Router” and routing in ReactJS

ReactJS programmers need knowledge of routing. More specifically, they need the knowledge of “React Router”. This standard library enables routing.

Routing makes it easy to navigate among views in various components in a ReactJS app. It allows one to change the browser URL, furthermore, it keeps the user interface synchronized with the URL.

11. The knowledge of synthetic events in React

You need a ReactJS developer to know about the browser’s native event and event handling. ReactJS provides useful wrappers that make up the event system of ReactJS. The “SyntheticEvent” wrapper is a key one of these wrappers, and developers should know about it.

12. Miscellaneous ReactJS skill areas

Look for ReactJS developers with skills in a few other aspects. They should know about “babel”, “createElement“, controlled and uncontrolled components, “mutable state”, “reducer”, templates, “classname”, “createclass”, “DOM elements”, “getInitialState”, HTML element, inline styles, internal states, lifecycle hooks, the difference from react native, etc. ReactJS developers should have significant debugging skills. Well-rounded skills in all of these areas can help your project significantly.

Hiring the right people for your development team is critical to the success of your project. Remember that you should not be aiming to hire the perfect ReactJS developer, but rather the right person for the job at hand.

With the help of this information and sample ReactJS interview questions, you can ensure that the hiring process goes smoothly and successfully – allowing you to hire a great programming team to get your project completed on time and on budget.

Frequently Asked Questions

Everything you need to know about ReactJS Interview Questions and Answers. Can’t find the answer you’re looking for?

1. Should I look for ReactJS developers that know other frameworks like AngularJS?

This depends on your project requirement. However, the knowledge of other popular frameworks like AngularJS, VueJS, etc. can be useful. Look for developers that have a thorough knowledge of the JavaScript ecosystem.

2. Do I need a ReactJS developer with both client-side and server-side programming skills?

You should look for both client-side and server-side programming experience when hiring a ReactJS developer. This gives you flexibility since such an experienced programmer can create apps that deliver functionality effectively.

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React coding tests

DevSkiller React coding tests are specially created by our expert team to help you find your next React developer. Our selection of React interview questions is designed to test the skills of React developers of any level of experience and make it easy for recruiters to filter out the best candidates.

DevSkiller React coding tests are powered by the RealLifeTesting ™ methodology, which offers a unique approach to developer testing. RealLifeTesting ™ provides valuable insight into your candidates’ skills by tasking them with real-world challenges and assessing the caliber of their responses. Finding your next React developer is easier than ever with DevSkiller React coding tests.

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DevSkiller React coding tests are powered by the RealLifeTesting ™ methodology. Unlike other methods for testing developers, the RealLifeTesting ™ methodology presents candidates with problems that mirror the real development work they do on a daily basis. This means that you can actively see how well someone will perform in their role, before they’ve even been given the job. Testing in this way offers recruiters valuable insight into the coding skills, problem-solving ability, and time-management skills of each React developer candidate, making for a more thorough and efficient screening process.

Key features of our React coding tests

Test your candidate’s coding skills and not just their academic knowledge
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React online tests available for all levels of skill

Skills covered in our React online tests

JavaScriptReact
Web development
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What to look for in React developers

React is a JavaScript library used for building user interfaces, specifically for single-page applications. Sometimes referred to as ReactJS, it is designed to easily integrate with existing code or other libraries. Its primary purpose is to simplify the development of web applications to make the process fast and simple, with less coding required. It is worth noting that React and React Native are not one and the same. To recruit a React developer you should be prepared to pay competitive rates. Here’s what else you need to know.

The most obvious skill to look for in a React developer is a thorough understanding of React itself. Beyond this, React uses JavaScript XML (JMX), an extension that allows HTML to be written in React. Many developers share the view that JMX is hard for junior developers to understand, so if your candidate can demonstrate proficiency in JMX it will be a big plus point.

Other desirable skills to look out for are experience with common front-end development tools and familiarity with code versioning tools. It will be helpful if they can demonstrate strong problem-solving and troubleshooting skills, as well as project management experience.

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react-svg library and styles are not applied

I have used the "react-svg" library to display SVG icons because I wanted to enter the address of the icon directly, but the problem is that I cannot implement my styles on the icon, like I want the size and change the color of the icon. I also want to change the color of the icon when the user hovers over it. Can I do this with this library? How? My codes:

tailwind-css

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How to Structure Customer Service Calls to Boost Satisfaction and Sales

Jonah Berger,
Grant Packard

Researchers found that service agents yield better results when they use warm language to start and end conversations, focusing on problem-solving in the middle.

We all know from our experience as customers that the things that salespeople say in a conversation affects our feelings and choices. A new study showed that the timing of language matters as well. By analyzing tens of thousands of moments or turns in service calls, researchers found that service agents get better customer satisfaction and purchase volume if they use warmer language at the start and finish of their interaction with a customer. Contrary to some common practices where a problem-solving mode is used right away, the results suggest that employees should use words that show competence only in the middle of a customer conversation.

Language plays a key role in almost every marketplace interaction. It’s how salespeople talk to prospects, leaders talk to teams, and customer service agents talk to customers. Recently, firms have been measuring and optimizing their language to manage the customer experience , automate service , and help make business decisions .

Jonah Berger is a professor at the Wharton School of the University of Pennsylvania and the author of Magic Words: What to Say to Get Your Way (Harper Business, 2023).
Yang Li is an associate professor of marketing at the Cheung Kong Graduate School of Business in Beijing, China.

Grant Packard is an associate professor of marketing at the Schulich School of Business at York University in Toronto, Canada.

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10 ReactJS Coding Challenge ( Coding Interview Prep )
Last month, I posted 2 ReactJS interview questions 🎤 - Advance Questions and Commonly Asked Question and got a significant welcome by dev.to community. 🤗 🥰 Many thanks to all of you! I decided to complete the package by creating codepen exercises 📝 and a video 📺 to follow up on that effort as one will need to code following the technical interview questions.
30 ReactJS Coding Interview Questions
Question 4. Develop a currency converter application that allows users to input an amount in one currency and convert it to another. For the sake of this challenge, you can use a hard-coded exchange rate. Take advantage of React state and event handlers to manage the input and conversion calculations. Question 5
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To master React, it's crucial to practice your skills with real-world scenarios and practical coding exercises. In this blog, we present 50 practical practice questions that range from beginner to advanced levels. These questions will help you hone your React skills and enhance your problem-solving abilities while working with code.
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Benefits of React Exercises. Interactive Quizzes: Engage in hands-on React quizzes. Progress Tracking: Monitor your learning journey. Skill Enhancement: Sharpen coding skills effectively. Flexible Learning: Practice at your own pace. Immediate Feedback: Receive instant results and feedback. Convenient Accessibility: Accessible online, anytime.
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3. Display a list in React. React code to print each item from the list on the page using Array.map() function to display each item on the page. The list of items is displayed using React JS through the following steps: Declare list of items as JS Array. Access each item using Array.map() Return JSX code in callback function for every item.
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React has been on a roll for a good 5 years now, and currently there is no end in sight. It keeps evolving. The average React Js Developer salary in USA is $125,000 per year or $64.10 per hour. Entry level positions start at $63,050 per year while most experienced workers make up to $195,000 per year. Follow along to learn most advanced React Interview Questions for your next tech interview.
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Tom is intelligent and learns quickly to get going on new projects. He approaches new challenges creatively, with a problem-solving mindset. He's particularly strong in React and has experience with hooks, functional, and class components. Tom has 12 years of experience working in technology, as well as 12 years working as a financial planner.
React coding exercises
Coding practice for React. Learn React programming, hooks, and components fast online. CodePractice. Languages. Explore Blog. Log In / Sign Up. Problem #249 - react (1 / 9) Render the string 'Hello' by returning it. xxxxxxxxxx . const HelloComponent = => {return} Run (ctrl+enter) This is the first problem. Next problem. Render a div containing ...
Real-World React Coding Challenges For Your Interview Preparation
4. Find The Bug. The (Lesser Known) Purpose of Coding Challenges in the Interview Process. 6 Tips for Coding Challenges in Job Interviews. Ask for Clarification. Talk While You're Coding (For Live Challenges) Clean Code Format. Use TypeScript and Write Tests If You Can. Add a README with Clear Instructions.
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React has exploded in popularity because its simple and declarative API produces highly-performant applications — and that momentum only continues to grow. In this guide, we break important React interview questions to ask down into three groups: Basic React Interview Questions. Intermediate React Interview Questions.
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In this guide, we break important React interview questions to ask down into three groups: Basic React Interview Questions; Intermediate React Interview Questions; Advanced React Interview Questions; Basic React Interview Questions. The following questions should test the candidate's basic knowledge of React and some of its core features. 1.
Crush Your React and Redux Interview: 20+ Top Questions and Strategies
It is often used in conjunction with other front-end technologies such as Redux for state management, and React Native for building mobile applications. Overall, React is a flexible and powerful tool for building complex and dynamic user interfaces, and it is widely used in the web development community. 2. What do you like and dislike about React?
10 React Interview Questions to Practice
10 React Interview Questions to Practice. 01/14/2022. By Kattie Thorndyke. React is an open-source JavaScript framework developed by Facebook. It's currently the most popular front-end framework in use today. Its success even convinced Facebook to develop React Native, another framework that uses React for cross-platform mobile development.
Acing the ReactJS Interview: Common Questions and Sample Answers
As React gains prominence, so does the demand for skilled ReactJS developers who can harness its power to craft seamless, interactive web applications. The ReactJS interview process is a pivotal gateway to these opportunities, testing candidates on their theoretical knowledge, coding prowess, and problem-solving acumen.
Online React Coding Test
React sample questions Example Question 1. ... Conduct React-focused live coding interviews in which candidates can exhibit their coding skills, problem-solving talents, and understanding of React fundamentals. 💡 To dig deeper: 5 ways to test developers' skills before hiring.
Top 60 React Interview Questions and Answers
The community continually contributes to the improvement and growth of the React ecosystem. 3. Explain the virtual DOM in React. The virtual DOM (Document Object Model) in React is a key concept that enhances performance. Instead of directly manipulating the actual browser DOM, React creates a virtual representation of it in memory.
20+ React Interview Questions with Answers
10. Contrast class and functional components. Among other interview questions on React.js, one more good point to clarify is to ask a candidate to describe these two components and tell when it is better to choose each of them. When it comes to the answer, expect to hear two differences: syntax and purpose.
React Challenges
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Top 50 React.Js Interview Questions
Top 50 React.Js Interview Questions - LeetCode Discuss. Level up your coding skills and quickly land a job. This is the best place to expand your knowledge and get prepared for your next interview.
40 ReactJS Advanced Interview Questions: Get Hired in 2024
React is quite the buzzword in the industry these days. As of now, React is the most popular front-end technology that more and more companies are using, and if you are preparing for a job interview, this is ReactJS interview questions tutorial is just the right one for you.Here's a comprehensive list of all the common ReactJS interview questions from basic to advanced levels that are ...
Mastering ReactJS Interview Questions and Answers: A ...
Remember, an interview is not just about providing the right answers; it's about showcasing your problem-solving skills, communication, and ability to apply your knowledge to practical situations.
32 ReactJS Interview Questions and Answers for 2024
Question 10: Explain what the Virtual DOM is in ReactJS and why it is necessary. Requirement: Expert ReactJS knowledge. Answer: Virtual DOM is one of the key concepts in React. A good candidate should be able to explain the problems with DOM manipulation and why the virtual DOM helps.
React online coding tests for recruiters and developers
Our selection of React interview questions is designed to test the skills of React developers of any level of experience and make it easy for recruiters to filter out the best candidates. ... problem-solving ability, and time-management skills of each React developer candidate, making for a more thorough and efficient screening process.
react-svg library and styles are not applied
I have used the "react-svg" library to display SVG icons because I wanted to enter the address of the icon directly, but the problem is that I cannot implement my styles on the icon, like I want the size and change the color of the icon. I also want to change the color of the icon when the user hovers over it. Can I do this with this library? How?
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Foreign diplomats react with horror to Biden's dismal debate
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