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Top 50 React Interview Questions and Answers for fresher and experience

React Interview Questions

So, here are the Top 50 React Interview Questions and Answers which are most likely to be asked by the interviewer. For your ease of access, I have categorized the React interview questions namely:
  • General React
  • React Component
  • React Redux
  • React Router

General React – React Interview Questions

1.  Differentiate between Real DOM and Virtual DOM.

Real DOM vs Virtual DOM

Real DOMVirtual  DOM
1. It updates slow.1. It updates faster.
2. Can directly update HTML.2. Can’t directly update HTML.
3. Creates a new DOM if element updates.3. Updates the JSX if element updates.
4. DOM manipulation is very expensive.4. DOM manipulation is very easy.
5. Too much of memory wastage.5. No memory wastage.

2. What is React?

  • React is a front-end JavaScript library developed by Facebook in 2011.
  • It follows the component based approach which helps in building reusable UI components.
  • It is used for developing complex and interactive web and mobile UI.
  • Even though it was open-sourced only in 2015, it has one of the largest communities supporting it.

3. What are the features of React? 

Major features of React are listed below:
  1. It uses the virtual DOM instead of the real DOM.
  2. It uses server-side rendering.
  3. It follows uni-directional data flow or data binding.

4. List some of the major advantages of React.

Some of the major advantages of React are:
  1. It increases the application’s performance
  2. It can be conveniently used on the client as well as server side
  3. Because of JSX, code’s readability increases
  4. React is easy to integrate with other frameworks like Meteor, Angular, etc
  5. Using React, writing UI test cases become extremely easy

5. What are the limitations of React?

Limitations of React are listed below:
  1. React is just a library, not a full-blown framework
  2. Its library is very large and takes time to understand
  3. It can be little difficult for the novice programmers to understand
  4. Coding gets complex as it uses inline templating and JSX


6. What is JSX?

JSX is a shorthand for JavaScript XML. This is a type of file used by React which utilizes the expressiveness of JavaScript along with HTML like template syntax. This makes the HTML file really easy to understand. This file makes applications robust and boosts its performance. Below is an example of JSX:
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render(){
    return(       
          
 
 
<div>
             
 
 
<h1> Hello World from Edureka!!</h1>
 
 
 
         </div>
 
 
 
    );
}

7. What do you understand by Virtual DOM? Explain its working.

A virtual DOM is a lightweight JavaScript object which originally is just the copy of the real DOM. It is a node tree that lists the elements, their attributes and content as Objects and their properties. React’s render function creates a node tree out of the React components. It then updates this tree in response to the mutations in the data model which is caused by various actions done by the user or by the system.
This Virtual DOM works in three simple steps.
  1. Whenever any underlying data changes, the entire UI is re-rendered in Virtual DOM representation.
  2. Then the difference between the previous DOM representation and the new one is calculated.
  3. Once the calculations are done, the real DOM will be updated with only the things that have actually changed. 

8. Why can’t browsers read JSX?

Browsers can only read JavaScript objects but JSX in not a regular JavaScript object. Thus to enable a browser to read JSX, first, we need to transform JSX file into a JavaScript object using JSX transformers like Babel and then pass it to the browser.

9. How different is React’s ES6 syntax when compared to ES5?

Syntax has changed from ES5 to ES6 in following aspects:
  1. require vs import
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    // ES5
    var React = require('react');
     
    // ES6
    import React from 'react';
  2. export vs exports
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    // ES5
    module.exports = Component;
     
    // ES6
    export default Component;
  3. component and function
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    // ES5
    var MyComponent = React.createClass({
        render: function() {
            return
     
     
    <h3>Hello Edureka!</h3>
     
     
     
    ;
        }
    });
     
    // ES6
    class MyComponent extends React.Component {
        render() {
            return
     
     
    <h3>Hello Edureka!</h3>
     
     
     
    ;
        }
    }
  4. props
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    // ES5
    var App = React.createClass({
        propTypes: { name: React.PropTypes.string },
        render: function() {
            return
     
     
    <h3>Hello, {this.props.name}!</h3>
     
     
     
    ;
        }
    });
     
    // ES6
    class App extends React.Component {
        render() {
            return
     
     
    <h3>Hello, {this.props.name}!</h3>
     
     
     
    ;
        }
    }
  5. state
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    // ES5
    var App = React.createClass({
        getInitialState: function() {
            return { name: 'world' };
        },
        render: function() {
            return
     
     
    <h3>Hello, {this.state.name}!</h3>
     
     
     
    ;
        }
    });
     
    // ES6
    class App extends React.Component {
        constructor() {
            super();
            this.state = { name: 'world' };
        }
        render() {
            return
     
     
    <h3>Hello, {this.state.name}!</h3>
     
     
     
    ;
        }
    }

10. How is React different from Angular?

React vs Angular

TOPICREACTANGULAR
1. ARCHITECTUREOnly the View of MVCComplete MVC
2. RENDERINGServer-side renderingClient-side rendering
3. DOMUses virtual DOMUses real DOM
4. DATA BINDINGOne-way data bindingTwo-way data binding
5. DEBUGGINGCompile time debuggingRuntime debugging
6. AUTHORFacebookGoogle

React Components – React Interview Questions

11. What do you understand from “In React, everything is a component.”

Components are the building blocks of a React application’s UI. These components split up the entire UI into small independent and reusable pieces. Then it renders each of these components independent of each other without affecting the rest of the UI.

12. Explain the purpose of render() in React.

Each React component must have a render() mandatorily. It returns a single React element which is the representation of the native DOM component. If more than one HTML element needs to be rendered, then they must be grouped together inside one enclosing tag such as <form>, <group>,<div> etc. This function must be kept pure i.e., it must return the same result each time it is invoked.
Name

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