25+ React js Interview Questions Answers pdf

Here are Top 25+ React Js Real Time Interview questions and their answers are given just below to them. These sample React Js questions are framed by experts. who train for Learn React Js Online to give you an idea of type of questions which may be asked in interview. We have taken full care to give correct answers for all the React Js questions. Do comment your thoughts. Happy Job Hunting!

1. What Is Reactjs?
React is an open source JavaScript front end UI library developed by Facebook for creating interactive, stateful & reusable UI components for web and mobile app. It is used by Facebook, Instagram and many more web apps.
ReactJS is used for handling view layer for web and mobile applications. One of React’s unique major points is that it perform not only on the client side, but also can be rendered on server side, and they can work together inter-operably.

2. Why Reactjs Is Used?
React is used to handle the view part of Mobile application and Web application.

3. Does Reactjs Use Html?
No, It uses JSX which is simiar to HTM.

4. When Reactjs Released?
March 2013

5. What Is Current Stable Version Of Reactjs?
Version: 15.5
Release on: April 7, 2017

6. What Are The Life Cycle Of Reactjs?
State/Property Updates

7. What Are The Feature Of Reactjs?
JSX: JSX is JavaScript syntax extension.
Components : React is all about components.
One direction flow: React implements one way data flow which makes it easy to reason about your app

8. What Are The Advantages Of Reactjs?
React uses virtual DOM which is JavaScript object. This will improve apps performance
It can be used on client and server side
Component and Data patterns improve readability.
Can be used with other framework also.

9. How To Embed Two Components In One Component?
import React from ‘react’;
class App extends React.Component {
render() {
return (


class Header extends React.Component {
render() {
return (

10. What Are The Advantages Of Using Reactjs?
Advantages of ReactJS:
React uses virtual DOM which is JavaScript object. This improves application performance as JavaScript virtual DOM is faster than the regular DOM.
React can be used on client and as well as server side too.
Using React increases readability and makes maintainability easier. Component, Data patterns improves readability and thus makes it easier for manitaing larger apps.
React can be used with any other framework (Backbone.js, Angular.js) as it is only a view layer.
React’s JSX makes it easier to read the code of our component. It’s really very easy to see the layout. How components are interacting, plugged and combined with each other in app.

11. What Are The Limitations Of Reactjs?
Limitations of ReactJS:
React is only for view layer of the app so we still need the help of other technologies to get a complete tooling set for development.
React is using inline templating and JSX. This can seem awkward to some developers.
The library of react is too large.
Learning curve for ReactJS may be steep.

12. How To Use Forms In Reactjs?
In React’s virtual DOM, HTML Input element presents an interesting problem. With the others DOM environment, we can render the input or textarea and thus allows the browser maintain its state that is (its value). we can then get and set the value implicitly with the DOM API.

In HTML, form elements such as <input>, <textarea>, and <select> itself maintain their own state and update its state based on the input provided by user .In React, components’ mutable state is handled by the state property and is only updated by setState().

HTML <input> and <textarea> components use the value attribute.
HTML <input> checkbox and radio components, checked attribute is used.
<option> (within <select>) components, selected attribute is used for select box.

13. How To Use Events In Reactjs?
React identifies every events so that it must have common and consistent behavior across all the browsers. Normally, in normal JavaScript or other frameworks, the onchange event is triggered after we have typed something into a Textfield and then “exited out of it”. In ReactJS we cannot do it in this way.

The explanation is typical and non-trivial:

*”<input type=”text” value=”dataValue”> renders an input textbox initialized with the value, “dataValue”.

When the user changes the input in text field, the node’s value property will update and change. However, node.getAttribute(‘value’) will still return the value used at initialization time that is dataValue.

Form Events:
onChange: onChange event watches input changes and update state accordingly.
onInput: It is triggered on input data
onSubmit: It is triggered on submit button.

Mouse Events:
onClick: OnClick of any components event is triggered on.
onDoubleClick: onDoubleClick of any components event is triggered on.
onMouseMove: onMouseMove of any components, panel event is triggered on.
onMouseOver: onMouseOver of any components, panel, divs event is triggered on.

Touch Events:
onTouchCancel: This event is for canceling an events.
onTouchEnd: Time Duration attached to touch of a screen.
onTouchMove: Move during touch device .
onTouchStart: On touching a device event is generated.

14. Give An Example Of Using Events?
import React from ‘react’;
import ReactDOM from ‘react-dom’;
var StepCounter = React.createClass({
getInitialState: function() { return {counter: this.props.initialCounter }; },
handleClick: function() {
this.setState({counter: this.state.counter + 1}); },
render: function() {
return <div onClick={this.handleClick}> OnClick Event, Click Here: {this.state.counter }</div>;
ReactDOM.render(< StepCounter initialCounter={7}/>, document.getElementById(‘content’));

15. Explain Various Flux Elements Including Action, Dispatcher, Store And View?
Flux can be better explained by defining its individual components:
Actions – They are helper methods that facilitate passing data to the Dispatcher.
Dispatcher – It is Central hub of app, it receives actions and broadcasts payloads to registered callbacks.
Stores – It is said to be Containers for application state & logic that have callbacks registered to the dispatcher. Every store maintains particular state and it will update when it is needed. It wakes up on a relevant dispatch to retrieve the requested data. It is accomplished by registering with the dispatcher when constructed. They are similar to model in a traditional MVC (Model View Controller), but they manage the state of many objects —  it does not represent a single record of data like ORM models do.
Controller Views – React Components grabs the state from Stores and pass it down through props to child components to view to render application.

16. What Is Flux Concept In Reactjs?
Flux is the architecture of an application that Facebook uses for developing client-side web applications. Facebook uses internally when working with React. It is not a framework or a library. This is simply a new technique that complements React and the concept of Unidirectional Data Flow.

Facebook dispatcher library is a sort of global pub/sub handler technique which broadcasts payloads to registered callbacks.

17. Give An Example Of Both Stateless And Stateful Components With Source Code?
Stateless and Stateful components

Stateless: When a component is “stateless”, it calculates state is calculated internally but it directly never mutates it. With the same inputs, it will always produce the same output. It means it has no knowledge of the past, current or future state changes.

var React = require(‘react’);
var Header = React.createClass({
render: function() {
return( <img src={this.props.imageSource} /> ); }
ReactDOM.render(<Header imageSource=”myImage.png”/>, document.body);

Stateful : When a component is “stateful”, it is a central point that stores every information in memory about the app/component’s state, do has the ability to change it. It has knowledge of past, current and potential future state changes. Stateful component change the state, using this.setState method.

var React = require(‘react’);
var Header = React.createClass({
getInitialState: function() {
return { imageSource: “header.png” };
changeImage: function() {
this.setState({imageSource: “changeheader.png”});
render: function() {
<img src={this.state.imageSource} onClick={this.changeImage.bind(this)} />
module.exports = Header;

18. Explain Basic Code Snippet Of Jsx With The Help Of A Practical Example?
Your browsers does not understand JSX code natively, we need to convert it to JavaScript first which can be understand by our browsers. We have aplugin which handles including Babel 5’s in-browser ES6 and JSX transformer called browser.js.

Babel will understand and recognize JSX code in <script type=”text/babel”></script> tags and transform/convert it to normal JavaScript code.

In case of production we will need to pre-compile our JSX code into JS before deploying to production environment so that our app renders faster.

<!DOCTYPE html>
<html lang=”en”>
<head><title>My First React JSX Example</title></head>
<div id=”hello-world”></div>
<script src=”https://fb.me/react-15.0.0.js”></script>
<script src=”https://fb.me/react-dom-15.0.0.js”></script>
<script src=”https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.34/browser.min.js”></script>
<script type=”text/babel”>
var HelloWorld = React.createClass({
render: function() {
return ( <p>Hello, World</p> )
ReactDOM.render( <HelloWorld/>, document.getElementById(‘hello-world’));

19. What Are The Advantages Of Using Jsx?
JSX is completely optional and its not mandatory, we don’t need to use it in order to use React, but it has several advantages and a lot of nice features in JSX.

JSX is always faster as it performs optimization while compiling code to vanilla JavaScript.
JSX is also type-safe, means it is strictly typed and most of the errors can be caught during compilation of the JSX code to JavaScript.
JSX always makes it easier and faster to write templates if we are familiar with HTML syntax.

20. What Is Reactjs-jsx?
JSX (JavaScript XML), lets us to build DOM nodes with HTML-like syntax. JSX is a preprocessor step which adds XML syntax to JavaScript.
Like XML, JSX tags have a tag name, attributes, and children JSX also has the same. If an attribute/property value is enclosed in quotes(“”), the value is said to be string. Otherwise, wrap the value in braces and the value is the enclosed JavaScript expression. We can represent JSX as <HelloWorld/>.

21. What Are Components In Reactjs?
React encourages the idea of reusable components. They are widgets or other parts of a layout (a form, a button, or anything that can be marked up using HTML) that you can reuse multiple times in your web application.

ReactJS enables us to create components by invoking the React.createClass() method features a render() method which is responsible for displaying the HTML code.

When designing interfaces, we have to break down the individual design elements (buttons, form fields, layout components, etc.) into reusable components with well-defined interfaces. That way, the next time we need to build some UI, we can write much less code. This means faster development time, fewer bugs, and fewer bytes down the wire.

22. How To Apply Validation On Props In Reactjs?
When the application is running in development mode, React will automatically check for all props that we set on components to make sure they must right correct and right data type.

For instance, if we say a component has a Message prop which is a string and is required, React will automatically check and warn if it gets invalid string or number or boolean objects. For performance reasons this check is only done on dev environments and on production it is disabled so that rendering of objects is done in fast manner .

Warning messages are generated easily using a set of predefined options such as:


23. What Are State And Props In Reactjs?
State is the place where the data comes from. We must follow approach to make our state as simple as possible and minimize number of stateful components.
For example, ten components that need data from the state, we should create one container component that will keep the state for all of them.
The state starts with a default value and when a Component mounts and then suffers from mutations in time (basically generated from user events).
A Component manages its own state internally, but—besides setting an initial state—has no business fiddling with the stateof its children. You could say the state is private.

import React from ‘react’;
import ReactDOM from ‘react-dom’;
var StepCounter = React.createClass({
getInitialState: function() {
return {counter: this.props.initialCount};
handleClick: function() {
this.setState({counter: this.state. counter + 1});
render: function() {
return <div onClick={this.handleClick}>{this.state.counter }</div>;
ReactDOM.render(< StepCounter initialCount={7}/>, document.getElementById(‘content’));

Props: They are immutable, this is why container component should define state that can be updated and changed. It is used to pass data down from our view-controller(our top level component).

When we need immutable data in our component we can just add props to reactDOM.render() function.

import React from ‘react’;
import ReactDOM from ‘react-dom’;
class PropsApp extends React.Component {

render() {
return (
ReactDOM.render(<PropsApp headerProperty = “Header from props…” contentProperty = “Content
from props…”/>, document.getElementById(‘app’));


24. What Is The Difference Between The State And Props In Reactjs?
Passes in from parent component.<PropsApp headerProperty = “Header from props…” contentProperty = “Content&nbsp;from props…”/>This properties are being read by PropsApp component and sent to ReactDOM View.

Created inside component by getInitialState.this.state reads the property of component and update its value it by this.setState() method and then returns to ReactDOM view.State is private within the component.