ReactsJs Interview Questions?

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Basic ReactJS Interview Questions & Answers

Below are some of the top ReactJS interview questions for freshers (with proper answers).

1. What is ReactJS?

ReactJS is an open-source, component-based front-end JavaScript library essential for the view layer of the application. It is supported by Facebook. 

ReactJS utilizes a virtual DOM-based structure to fill in the information in HTML DOM. The virtual DOM works quickly, possessing a way that it only modifies single DOM components rather than refreshing the complete DOM every time.

The React application comprises several components, each answerable for outputting a small, reusable part of HTML. Components can be embedded inside other components to permit complex applications to work out of basic building blocks. 

A component can also support an internal state. For instance, a TabList component can save a variable compared to the directly open tab.

2. What are the benefits of ReactJS?

The main advantages of ReactJS are as follows:

a) Easy to learn

ReactJS is not a core but a JS library that takes together objects and targets a specific thing to complete it effectively. It is the view controller in the MVC structure. Any developer who learns JavaScript can interpret React, master its fundamentals, and develop a fantastic app.

b) Reusable Components

Reusing components is the primary feature of React JS. Even Facebook has executed React as it supports the reuse of framework components. A developer can begin with numerous components like a checkbox, button, etc. 

We develop wrapper components made out of those smaller components. Then, we write higher-level wrapper components. It goes on like that until we have this one root component, and that component is our app.

c) Virtual DOM

When we are about to build a web application with high customer connection and view updates, similar to the new form-maker on JotForm 4.0, we have to consider the available execution problem. 

Updating DOM is generally the bottleneck concerning web execution. React attempts to understand this issue by utilizing virtual DOM and a DOM kept in memory.

d) Flux and Redux architecture

Facebook developed Flux architecture for its various web applications. It is like React components in its unidirectional flow. This structure has action creators that facilitate making action from method parameters. It also supports a library for these methods. 

All these actions are supported together by a focal dispatcher to refresh stores. All views are restored according to the stores. There is likewise Redux, which is an upgraded version of flux architecture. 

It has an individual store which is not needed in flux. Redux also allows a feature where middleware can be characterized to capture dispatched actions.

e) SEO-Friendly

3. What is Create React App?

This is one of the frequently asked React interview questions for freshers. Find the answer below:

Create react app is a React app standard generator developed by Facebook. It supports a development setting configured for usability with minimal setup. 

Here are some of the most important things to know about “create react app”:

  1. It involves ES6 and JSX transpilation
  2. Consists of a Dev server with hot module reloading
  3. Used for Code linting
  4. Can contain CSS auto-prefixing
  5. It can develop the script with JS, CSS, picture packaging, and sourcemaps
  6. Used for the Jest testing system.

4. What are the features of ReactJS?

There are several features of ReactJS as follows:

a) Virtual DOM

In React, for each DOM object, there is a corresponding “virtual DOM object.” A virtual DOM object is a definition of a DOM object and makes a virtual replica of the initial DOM. 

It is a one-way data-binding; therefore, controlling the virtual DOM is faster than updating the initial DOM since nothing gets drawn onscreen.

b) JSX

JSX represents JavaScript XML. It is a JavaScript syntax extension. It is an XML or HTML like syntax utilized by ReactJS. 

This syntax is handled in JavaScript calls of React framework. It broadens the ES6 so HTML-like content can exist together with JavaScript react code. It is not necessary to utilize JSX, but it is endorsed to use it in ReactJS.

c) Simplicity

ReactJS uses the JSX file, which makes the application simple to program and understand. 

We learn that ReactJS is a component-based technique that establishes the code reusable as our need. This creates it simple to use and learn.

d) One-way Data Binding

One-way data binding defines that data flows only in one direction through the entire application. This provides better control over it. The data is moved from the parent component to the child component through read-only props. 

These props cannot be sent back to the parent component. This is how one-way data binding works. Although, the child component can interact with the parent component to update the state via callback functions.

e) React Native

React Native is a custom performed for React, directly comparable to React DOM on the web. It uses native components rather than web components like React as building blocks. 

It can start with React Native, and we must know the essential React concepts, like JSX, components, state, and props. It also provides an approach to these platform features, from changing the React program to working on iOS and Android.

5. What are the disadvantages of ReactJS?

The disadvantages of ReactJS are as follows:

a) Dynamic Technology

One of the disadvantages of ReactJS is that it keeps on changing with time. Therefore, it is a constant learning technique for the developers, who have to understand new methods of doing things that come with uncertain circumstances.

b) Average Documentation

It is another disadvantage of constantly updating technologies. There is a lack of suitable documentation because of the fast updating of React technologies.

c) JSX as a barrier

ReactJS utilizes JSX, which is a syntax extension that enables the combining of HTML and JavaScript. This method is helpful, but some development community members treat JSX as a barrier, especially the new developers. In addition, developers argue about the learning curve complexity of JSX.

d) View Part

Only UI layers of the app get covered by ReactJS. Therefore, choosing a few other technologies is necessary to get a complete collection of tooling for the project’s development.

6. How to install ReactJS?

ReactJS is a JavaScript library included in a single file react-<version>.js that can be contained in any HTML page. The developers also generally install the React DOM library react-dom-<version>.js along with the main React file:


!DOCTYPE html>




 <script type="text/javascript" src="/path/to/react.js"></script>

 <script type="text/javascript" src="/path/to/react-dom.js"></script>

 <script type="text/javascript">

 // Use react JavaScript program here or in an individual file




To obtain the JavaScript files, go to the of the official React documentation. 

React also provides JSX syntax. JSX is an extension generated by Facebook that inserts XML syntax into JavaScript. It can use JSX we require to contain the Babel library and change <script type=”text/javascript”> to <script type=”text/babel”> for translating JSX to Javascript program.

<!DOCTYPE html>




 <script type="text/javascript" src="/path/to/react.js"></script>

 <script type="text/javascript" src="/path/to/react-dom.js"></script>

 <script src=""></script>

 <script type="text/babel">

 // Use react JSX program here or in an individual file




Installing ReactJS via npm

We can also install React using npm by doing the following:

npm install –save react react-dom

To use React in our JavaScript project, we can do the following:

var React = require(‘react’);

var ReactDOM = require(‘react-dom’);

ReactDOM.render(<App/>, …);

Installing React via Yarn

Facebook released its package manager, Yarn, which can also be used to install React. After installing Yarn, we are just required to run this command:

yarn add react react-dom

We can then use React in our project precisely as if we had installed React via npm.

7. What is the difference between ReactJS and AngularJS?

This is among the most common ReactJS interview questions. We have curated a simple tabular comparison to help you understand the differences between ReactJS vs AngularJS.

                             ReactJS                AngularJS
            DefinitionReactJS is an open-source JavaScript library. It is used to develop a user interface for a single-page application. It is answerable only for the view layer of the application. AngularJS is an open-source JavaScript framework that can develop a powerful web application. It is an MVC framework that works with the MVC platform, where it facilitates development by giving a dependable solution.
Developed ByFacebookGoogle
            Data BindingReactJS supports one-way binding. It provides singular behavior for your application. One-way data-binding defines some changes we create to the model that influence the view, but not the other way around. In these methods, the data only flows in one direction.AngularJS uses a two-way data binding, which links the Document Object Model (DOM) values to model data. It defines if a new value is supported in the app for user interaction with the field. It will appear in the update of both the view and the model.
            SimplicityReact is not simple, and it takes some time to start a project.Angular is easy and simple to understand. However, its inherent complexity sometimes confuses.
        DOM UsageReact uses a virtual DOM. A virtual DOM is a secure version of the DOM. We can modify any element rapidly without needing to render the whole DOM. Angular uses the browser’s DOM.
      Application StructureReact is not an MVC-like Framework. It is like a view-based library. React does not demand its client or developer to use some particular application structure.Angular is a complete-featured MVC Framework. Angular MVC-based structure always permits the application to split into three associated parts so that they can be simply manipulated.

8. What is the difference between ReactJS and React Native?

Here is the ReactJS and React Native comparison to understand the differences between the two:

9. Which browsers does ReactJS support?

It supports all the popular and most used web browsers like Google Chrome, Firefox, Mozilla, Microsoft Edge, etc. It doesn’t support browsers built without ES5 methods, like Internet Explorer.

10. What is JSX in React?

JSX is a preprocessor phase that inserts XML syntax into JavaScript. We can use React without JSX, yet JSX makes React a lot more classic.

It is just a similar XML. JSX tags contain a tag name, attributes, and children. If an attribute value is placed within quotes, the value is a string. Alternatively, wrap the value in braces, and the value is the confined JavaScript expression.

JSX gives syntactic sugar to React.createElement(component, props, …children) function.

11. What are the benefits of using JSX in React?

The advantages of JSX are as follows:

  1. It is always quick as it implements optimization while assembling a program to vanilla JavaScript.
  2. JSX is additionally type-safe, which defines it as carefully composed, and most of the errors can be found during the compilation of the JSX code to JavaScript.
  3. It consistently makes writing templates simpler and quicker if we are simple with HTML syntax.

12. What is the difference between DOM and Virtual DOM in ReactJS?

Another common question asked in the ReactJS job interviews is about the comparison between DOM and VDOM. Here is the answer:


DOM represents the Document Object Model. It is also known as HTML DOM, an abstraction of a structured code known as HTML for web developers. DOM and HTML code are associated, as the elements of HTML are called nodes of DOM. 

It describes a structure where users can create, alter, modify documents, and present the content. Therefore while HTML is text, DOM is an in-memory definition of this content.

– Virtual DOM (VDOM)

VDOM in ReactJS is a method that syncs the virtual user interface with the real document object model. It is not a dedicated technology but a pattern that helps in handling the events, updating manual DOM, as well as manipulating the attributes. 

Interview Questions for You to Prepare for Jobs

13. What is the use of keys in ReactJS?

Keys recognize unique virtual DOM elements with their related data driving the UI. It supports React to develop rendering by recycling existing DOM components. Keys must be unique numbers or strings. Rather than re-rendering with keys, React only re-orders the components. It enhances the application’s execution.

14. What is Relay in ReactJS?

Relay is a JavaScript framework supporting an information layer and user-server connection to web applications utilizing the React view layer.

15. What is the difference between createElement and cloneElement?

JSX components are transpiled to React.createElement() functions to make React elements that will be used for the object definition of UI. 

Whereas, cloneElement is used to clone an element and pass it new props.

16. How to create components in React?

It is one of the frequently asked React interview questions for freshers. You must know that there are two approaches to creating a component in ReactJS:

Functional Components

It is the simplest method of creating the React component. These pure JavaScript functions obtain the props object as the first argument and return the react component. 

function Hello({ message }) {
return <h1>{`Hello, ${message}`}</h1>


Class components

We can use ES6 class to create a component. The function component can be written as:

class App extends React.Component {

render() {

return <h2>{`Hii, ${this.props.message}`}</h2> 



17. What is a React State?

States are the soul of React elements. These are the source of information and should be maintained as simply as feasible. 

ReactJS states are the objects which decide the component’s rendering and behavior. They are mutable, unlike the props, and generate powerful and interactive elements. They are accessed through this.state().

18. What are React Props?

Props are inputs to components. They are read-only components that should be kept immutable. ReactJS Props are used to pass information and methods from a parent component to a child component. 

19. What are Default Props?

In ReactJS, defaultProps authorize us to set default, or fallback, values for our component props. defaultProps are helpful when we call components from various views with fixed props, but in some views, we are required to pass several values.

Example of React Default Props

class MyApp extends React.Component {...}

MyApp.defaultProps = {

 randomObject: {},



20. What is the difference between state and props in ReactJS?

Here is the tabular comparison between React state and props:

21. What is setState() in ReactJS?

The primary method using which we create UI updates to our React applications is through a call to the setState() function. This function will execute a shallow merge between the new state that we give and the previous state and trigger a re-render of our component and all decedents.

22. What are the Parameters of setState()?

There are two parameters of setstate() as follows:

  1. Updater: It can be an object with several key-value pairs that should be combined into the state or a function that restores such an object.
  2. Call-back (optional): The function implemented after setState() is implemented effectively. Because calls to setState() are not ensured by React to be atomic, this can be beneficial if we need to implement some action after we are positive that setState() has been performed effectively.

23. What is the arrow function in React?

An arrow function is also known as the ‘fat arrow function (=>).’ It enables binding the context of components properly because auto-binding is not possible by default in ES6. In addition, it creates it easier to work with higher-order components.  


  1. Without using Arrow functions
render() {
 return (<MyInput onChange={this.handleChange.bind(this) }  />)

  1. Using Arrow Functions
render() {
return (<MyInput onChange={(e)=>this.handleOnChange(e) }  />)

24. What is the context in ReactJS?

The context supports a method to pass information through the component tree without giving props down manually at each level.

For example, authenticated customers, locale preference, and UI theme should be acquired in the application by many components.

const {Provider, User} = React.createContext(defaultValue)

25. What are React Mixins?

Mixins are an approach to separate elements from having standard functionality. It should not be used and can be replaced with higher-order components or decorators.

26. What is the context.consumer in React?

A consumer is a React element that subscribes to text changes. It needed a function as a child that accepts the current context value as a parameter and returns a react node. The value parameter passed to the function will be similar to the nearest provider’s value props for this context above in the tree.


  {value => /* render something depend on the context value */}

27. What is React Fiber?

Fiber is the new reconciliation engine or re-executing the basic algorithm in React v16. The objective of React Fiber is to expand its appropriateness for fields like animation, design, gestures, and the ability to pause, abort, or reuse work and assign priority to multiple types of updates; and new concurrency primitives.

28. What is React reconciliation?

When a component’s props or state changes, React determines whether an actual DOM update is essential by contrasting the recently returned element with the previously rendered one. 

When they are not similar, they will update the DOM. This process is known as reconciliation in ReactJS.

29. What are Pure Components?

Pure components are the easiest and quickest components that can be written. They can restore any component which has a render(). These components upgrade the program’s simplicity and the application’s execution.