Best 50 React Interview Questions and Answers in 2019

Best 50 React Interview Questions and Answers in 2019
If you are an aspiring front end developer and preparing for interviews, then this article is for you. Best 50 React Interview Questions is the perfect guide for you to learn all the concepts required to clear a React interview.

But before starting with the React Interview Questions, let’s take a quick look at React’s demand and status in the market.

As of today, there are about 1,000 contributors on Github. Unique features like Virtual DOM and reusable components grab the attention of front end developers. Despite being just a library for ‘View’ in MVC (Model-View-Controller), it’s giving strong competition to full-blown frameworks like Angular, Meteor, Vue, etc. Check out the belo2w graph which shows the trend of popular JS frameworks:


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 Interview Questions

1. Differentiate between Real DOM and Virtual DOM.


2. What is React?

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:

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:

// ES5
var React = require('react');
 
// ES6
import React from 'react';
// ES5
module.exports = Component;
 
// ES6
export default Component;
// ES5
var MyComponent = React.createClass({
    render: function() {
        return
 
 
<h3>Hello Edureka!</h3>
 
 
 
;
    }
});
 
// ES6
class MyComponent extends React.Component {
    render() {
        return
 
 
<h3>Hello Edureka!</h3>
 
 
 
;
    }
}
// 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>
 
 
 
;
    }
}
// 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 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.


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

We can embed components into one in the following way:

class MyComponent extends React.Component{
    render(){
        return(         
             
 
 
<div>
               
                 
 
 
<h1>Hello</h1>
 
 
 
                <Header/>
            </div>
 
 
 
        );
    }
}
class Header extends React.Component{
    render(){
        return
 
 
<h1>Header Component</h1>
 
 
 
    
   };
}
ReactDOM.render(
    <MyComponent/>, document.getElementById('content')
);

14. What is Props?

Props is the shorthand for Properties in React. They are read-only components which must be kept pure i.e. immutable. They are always passed down from the parent to the child components throughout the application. A child component can never send a prop back to the parent component. This help in maintaining the unidirectional data flow and are generally used to render the dynamically generated data.


15. What is a state in React and how is it used?

States are the heart of React components. States are the source of data and must be kept as simple as possible. Basically, states are the objects which determine components rendering and behavior. They are mutable unlike the props and create dynamic and interactive components. They are accessed via this.state().


16. Differentiate between states and props.

17. How can you update the state of a component?

State of a component can be updated using this.setState().

class MyComponent extends React.Component {
    constructor() {
        super();
        this.state = {
            name: 'Maxx',
            id: '101'
        }
    }
    render()
        {
            setTimeout(()=>{this.setState({name:'Jaeha', id:'222'})},2000)
            return (             
                    
 
 
<div>
                   
                      
 
 
<h1>Hello {this.state.name}</h1>
 
 
 
                 
                      
 
 
<h2>Your Id is {this.state.id}</h2>
 
 
 
                   </div>
 
 
 
            );
        }
    }
ReactDOM.render(
    <MyComponent/>, document.getElementById('content')
);

18. What is arrow function in React? How is it used?

Arrow functions are more of brief syntax for writing the function expression. They are also called ‘fat arrow‘ (=>) the functions. These functions allow to bind the context of the components properly since in ES6 auto binding is not available by default. Arrow functions are mostly useful while working with the higher order functions.

//General way
render() {   
    return(
        <MyInput onChange={this.handleChange.bind(this) } />
    );
}
//With Arrow Function
render() { 
    return(
        <MyInput onChange={ (e) => this.handleOnChange(e) } />
    );
}

19. Differentiate between stateful and stateless components.

20. What are the different phases of React component’s lifecycle?

There are three different phases of React component’s lifecycle:

21. Explain the lifecycle methods of React components in detail.

Some of the most important lifecycle methods are:

22. What is an event in React?

In React, events are the triggered reactions to specific actions like mouse hover, mouse click, key press, etc. Handling these events are similar to handling events in DOM elements. But there are some syntactical differences like:

The event argument contains a set of properties, which are specific to an event. Each event type contains its own properties and behavior which can be accessed via its event handler only.

23. How do you create an event in React?

class Display extends React.Component({   
    show(evt) {
        // code  
    },  
    render() {     
        // Render the div with an onClick prop (value is a function)        
        return (           
           
 
 
<div onClick={this.show}>Click Me!</div>
 
 
 
        );   
    }
});

24. What are synthetic events in React?

Synthetic events are the objects which act as a cross-browser wrapper around the browser’s native event. They combine the behavior of different browsers into one API. This is done to make sure that the events show consistent properties across different browsers.


25. What do you understand by refs in React?

Refs is the short hand for References in React. It is an attribute which helps to store a reference to a particular React element or component, which will be returned by the components render configuration function. It is used to return references to a particular element or component returned by render(). They come in handy when we need DOM measurements or to add methods to the components.

class ReferenceDemo extends React.Component{
     display() {
         const name = this.inputDemo.value;
         document.getElementById('disp').innerHTML = name;
     }
render() {
    return(       
          
 
 
<div>
            Name: <input type="text" ref={input => this.inputDemo = input} />
            <button name="Click" onClick={this.display}>Click</button>            
             
 
 
<h2>Hello <span id="disp"></span> !!!</h2>
 
 
 
        </div>
 
 
 
    );
   }
 }

26. List some of the cases when you should use Refs.

Following are the cases when refs should be used:

27. How do you modularize code in React?

We can modularize code by using the export and import properties. They help in writing the components separately in different files.

//ChildComponent.jsx
export default class ChildComponent extends React.Component {
    render() {
        return(          
              
 
 
<div>
              
                 
 
 
<h1>This is a child component</h1>
 
 
 
            </div>
 
 
 
        );
    }
}
 
//ParentComponent.jsx
import ChildComponent from './childcomponent.js';
class ParentComponent extends React.Component {   
    render() {       
        return(          
             
 
 
<div>              
                <App />         
            </div>
 
 
 
        
        ); 
    }
}

28. How are forms created in React?

React forms are similar to HTML forms. But in React, the state is contained in the state property of the component and is only updated via setState(). Thus the elements can’t directly update their state and their submission is handled by a JavaScript function. This function has full access to the data that is entered by the user into a form.

handleSubmit(event) {
    alert('A name was submitted: ' + this.state.value);
    event.preventDefault();
}
 
render() {
    return (       
         
 
 
<form onSubmit={this.handleSubmit}>
            <label>
                Name:
                <input type="text" value={this.state.value} onChange={this.handleSubmit} />
            </label>
            <input type="submit" value="Submit" />
        </form>
 
 
 
    );
}

29. What do you know about controlled and uncontrolled components?

30. What are Higher Order Components(HOC)?

Higher Order Component is an advanced way of reusing the component logic. Basically, it’s a pattern that is derived from React’s compositional nature. HOC are custom components which wrap another component within it. They can accept any dynamically provided child component but they won’t modify or copy any behavior from their input components. You can say that HOC are ‘pure’ components.


31. What can you do with HOC?

HOC can be used for many tasks like:

32. What are Pure Components?

Pure components are the simplest and fastest components which can be written. They can replace any component which only has a render(). These components enhance the simplicity of the code and performance of the application.

33. What is the significance of keys in React?

Keys are used for identifying unique Virtual DOM Elements with their corresponding data driving the UI. They help React to optimize the rendering by recycling all the existing elements in the DOM. These keys must be a unique number or string, using which React just reorders the elements instead of re-rendering them. This leads to increase in application’s performance.


React Redux – React Interview Questions

34. What were the major problems with MVC framework?

Following are some of the major problems with MVC framework:

35. Explain Flux.

Flux is an architectural pattern which enforces the uni-directional data flow. It controls derived data and enables communication between multiple components using a central Store which has authority for all data. Any update in data throughout the application must occur here only. Flux provides stability to the application and reduces run-time errors.

36. What is Redux?

Redux is one of the hottest libraries for front-end development in today’s marketplace. It is a predictable state container for JavaScript applications and is used for the entire applications state management. Applications developed with Redux are easy to test and can run in different environments showing consistent behavior.


37. What are the three principles that Redux follows?

38. What do you understand by “Single source of truth”?

Redux uses ‘Store’ for storing the application’s entire state at one place. So all the component’s state are stored in the Store and they receive updates from the Store itself. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.

39. List down the components of Redux.

Redux is composed of the following components:


40. Show how the data flows through Redux?

41. How are Actions defined in Redux?

Actions in React must have a type property that indicates the type of ACTION being performed. They must be defined as a String constant and you can add more properties to it as well. In Redux, actions are created using the functions called Action Creators. Below is an example of Action and Action Creator:

function addTodo(text) {
       return {
                type: ADD_TODO,    
                 text   
    }
}

42. Explain the role of Reducer.

Reducers are pure functions which specify how the application’s state changes in response to an ACTION. Reducers work by taking in the previous state and action, and then it returns a new state. It determines what sort of update needs to be done based on the type of the action, and then returns new values. It returns the previous state as it is, if no work needs to be done.


43. What is the significance of Store in Redux?

A store is a JavaScript object which can hold the application’s state and provide a few helper methods to access the state, dispatch actions and register listeners. The entire state/ object tree of an application is saved in a single store. As a result of this, Redux is very simple and predictable. We can pass middleware to the store to handle the processing of data as well as to keep a log of various actions that change the state of stores. All the actions return a new state via reducers.


44. How is Redux different from Flux?

45. What are the advantages of Redux?

Advantages of Redux are listed below:

React Router – React Interview Questions

46. What is React Router?

React Router is a powerful routing library built on top of React, which helps in adding new screens and flows to the application. This keeps the URL in sync with data that’s being displayed on the web page. It maintains a standardized structure and behavior and is used for developing single page web applications. React Router has a simple API.


47. Why is switch keyword used in React Router v4?

Although a <div> is used to encapsulate multiple routes inside the Router. The ‘switch’ keyword is used when you want to display only a single route to be rendered amongst the several defined routes. The <switch> tag when in use matches the typed URL with the defined routes in sequential order. When the first match is found, it renders the specified route. Thereby bypassing the remaining routes.


48. Why do we need a Router in React?

A Router is used to define multiple routes and when a user types a specific URL, if this URL matches the path of any ‘route’ defined inside the router, then the user is redirected to that particular route. So basically, we need to add a Router library to our app that allows creating multiple routes with each leading to us a unique view.

<switch>
    <route exact path=’/’ component={Home}/>
    <route path=’/posts/:id’ component={Newpost}/>
    <route path=’/posts’   component={Post}/>
</switch>

49. List down the advantages of React Router.

Few advantages are:

50. How is React Router different from conventional routing?

I hope this set of React Interview Questions and Answers will help you in preparing for your interviews. All the best!

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