The React.js is an open-source JavaScript library that is used for making user interfaces exactly for single-page applications. It’s used for treatment the view layer for web and mobile apps. The React also allows us to create refillable UI components. The React was first formed by Jordan Walke, a software engineer working for Facebook. The React first organized on Facebook’s newsfeed in 2011 and on Instagram.com in 2012.
The React allows developers to make large web applications that can change data, without reloading the page. The main determination of React is to be fast, scalable, and simple. It works only on user lines in the application. This agrees to the view in the MVC template. It can be used with a mixture of other JavaScript libraries or frameworks, such as Angular JS in MVC.
The React JS is also called just to react or React.js.
Interview questions and answers for React JS
1) What is React?
React is a declarative, efficient, flexible open source front-end JavaScript library developed by Facebook in 2011. It follows the component-based approach for building reusable UI components, especially for single page application. It is used for developing interactive view layer of web and mobile apps. It was created by Jordan Walke, a software engineer at Facebook. It was initially deployed on Facebook’s News Feed section in 2011 and later used in its products like WhatsApp & Instagram.
2) What are the features of React?
React framework gaining quick popularity as the best framework among web developers. The main features of React are:
- JSX
- Components
- One-way Data Binding
- Virtual DOM
- Simplicity
- Performance
3) What are the most crucial advantages of using react?
Following is a list of the most crucial advantages of using React:
- React is easy to learn and use
- React comes with good availability of documentation, tutorials, and training resources. It is easy for any developer to switch from JavaScript background to React and easily understand and start creating web apps using react. Anyone with little knowledge of JavaScript can start building web applications using React.
- React follows the MVC architecture.
- React is the V (view part) in the MVC (Model-View-Controller) architecture model and is referred to as “one of the JavaScript frameworks.” It is not fully featured but has many advantages of the open-source JavaScript User Interface (UI) library, which helps execute the task in a better manner.
- React uses Virtual DOM to improve efficiency.
- React uses virtual DOM to render the view. The virtual DOM is a virtual representation of the real DOM. Each time the data changes in a react app, a new virtual DOM gets created. Creating a virtual DOM is much faster than rendering the UI inside the browser. Therefore, with the use of virtual DOM, the efficiency of the app improves. That’s why React provides great efficiency.
- Creating dynamic web applications is easy.
- In React, creating a dynamic web application is much easier. It requires less coding and gives more functionality. It uses JSX (JavaScript Extension), which is a particular syntax letting HTML quotes and HTML tag syntax to render particular subcomponents.
- React is SEO-friendly.
- React facilitates a developer to develop an engaging user interface that can be easily navigated in various search engines. It also allows server-side rendering, which is also helpful to boost the SEO of your app.
- React allows reusable components.
- React web applications are made up of multiple components where each component has its logic and controls. These components provide a small, reusable piece of HTML code as an output that can be reused wherever you need them. The code reusability helps developers to make their apps easier to develop and maintain. It also makes the nesting of the components easy and allows developers to build complex applications of simple building blocks. The reuse of components also increases the pace of development.
- Support of handy tools
- React provides a lot of handy tools that can make the task of the developers understandable and easier. Use these tools in Chrome and Firefox dev extension, allowing us to inspect the React component hierarchies in the virtual DOM. It also allows us to select the particular components and examine and edit their current props and state.
- React has a rich set of libraries.
- React has a huge ecosystem of libraries and provides you the freedom to choose the tools, libraries, and architecture for developing the best application based on your requirement.
- Scope for testing the codes
- React web applications are easy to test. These applications provide a scope where the developer can test and debug their codes with the help of native tools.
4) What are the biggest limitations of React?
Following is the list of the biggest limitations of React:
React is just a library. It is not a complete framework.
It has a huge library which takes time to understand.
It may be difficult for the new programmers to understand and code.
React uses inline templating and JSX, which may be difficult and act as a barrier. It also makes the coding complex.
5) What is JSX?
JSX stands for JavaScript XML. It is a React extension which allows writing JavaScript code that looks similar to HTML. It makes HTML file easy to understand. The JSX file makes the React application robust and boosts its performance. JSX provides you to write XML-like syntax in the same file where you write JavaScript code, and then preprocessor (i.e., transpilers like Babel) transform these expressions into actual JavaScript code. Just like XML/HTML, JSX tags have a tag name, attributes, and children.
Example
class App extends React.Component {
render() {
return(
<div>
<h1>Hello JavaTpoint</h1>
</div>
)
}
}
In the above example, text inside <h1> tag return as JavaScript function to the render function. After compilation, the JSX expression becomes a normal JavaScript function, as shown below.
React.createElement(“h1”, null, “Hello JavaTpoint”);
6) Why can’t browsers read JSX?
Browsers cannot read JSX directly because they can only understand JavaScript objects, and JSX is not a regular JavaScript object. Thus, we need to transform the JSX file into a JavaScript object using transpilers like Babel and then pass it to the browser.
7) Why we use JSX?
It is faster than regular JavaScript because it performs optimization while translating the code to JavaScript.
Instead of separating technologies by putting markup and logic in separate files, React uses components that contain both.
T is type-safe, and most of the errors can be found at compilation time.
It makes easier to create templates.
8) What do you understand by Virtual DOM?
A Virtual DOM is a lightweight JavaScript object which is an in-memory representation of real DOM. It is an intermediary step between the render function being called and the displaying of elements on the screen. It is similar to a node tree which lists the elements, their attributes, and content as objects and their properties. The render function creates a node tree of the React components and then updates this node tree in response to the mutations in the data model caused by various actions done by the user or by the system.
9) Explain the working of Virtual DOM.
Virtual DOM works in three steps:
1. Whenever any data changes in the React App, the entire UI is re-rendered in Virtual DOM representation.
2. Now, the difference between the previous DOM representation and the new DOM is calculated.
3. Once the calculations are completed, the real DOM updated with only those things which are changed.
How Reacts ES6 syntax is different from ES5 syntax?
The Reacts ES6 syntax has changed from ES5 syntax in the following aspects.
Require vs. Import
// ES5
var React = require('react');
// ES6
import React from 'react';
exports vs. export
// ES5
module.exports = Component;
// ES6
export default Component;
component and function
// ES5
var MyComponent = React.createClass({
render: function() {
return(
<h3>Hello JavaTpoint</h3>
);
}
});
// ES6
class MyComponent extends React.Component {
render() {
return(
<h3>Hello Javatpoint</h3>
);
}
}
props
// 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>
);
}
}
state
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>
);
}
}
14) What do you understand from “In React, everything is a component?”
In React, components are the building blocks of React applications. These components divide the entire React application’s UI into small, independent, and reusable pieces of code. React renders each of these components independently without affecting the rest of the application UI. Hence, we can say that, in React, everything is a component.
15) Explain the purpose of render() in React.
It is mandatory for each React component to have a render() function. Render function is used to return the HTML which you want to display in a component. If you need to rendered more than one HTML element, you need to grouped together inside single enclosing tag (parent tag) such as <div>, <form>, <group> etc. This function returns the same result each time it is invoked.
Example: If you need to display a heading, you can do this as below.
import React from ‘react’
class App extends React.Component {
render (){
return (
<h1>Hello World</h1>
)
}
}
export default App
Points to Note:
Each render() function contains a return statement.
The return statement can have only one parent HTML tag.
16) How can you embed two or more components into one?
You can embed two or more components into the following way:
import React from ‘react’
class App extends React.Component {
render (){
return (
<h1>Hello World</h1>
)
}
}
class Example extends React.Component {
render (){
return (
<h1>Hello JavaTpoint</h1>
)
}
}
export default App
17) What is Props?
Props stand for “Properties” in React. They are read-only inputs to components. Props are an object which stores the value of attributes of a tag and work similar to the HTML attributes. It gives a way to pass data from the parent to the child components throughout the application.
It is similar to function arguments and passed to the component in the same way as arguments passed in a function.
Props are immutable so we cannot modify the props from inside the component. Inside the components, we can add attributes called props. These attributes are available in the component as this.props and can be used to render dynamic data in our render method.
18) What is a State in React?
The State is an updatable structure which holds the data and information about the component. It may be changed over the lifetime of the component in response to user action or system event. It is the heart of the react component which determines the behavior of the component and how it will render. It must be kept as simple as possible.
Let’s create a “User” component with “message state.”
import React from 'react'
class User extends React.Component {
constructor(props) {
super(props)
this.state = {
message: 'Welcome to JavaTpoint'
}
}
render() {
return (
<div>
<h1>{this.state.message}</h1>
</div>
)
}
}
export default User
19) How can you update the State of a component?
We can update the State of a component using this.setState() method. This method does not always replace the State immediately. Instead, it only adds changes to the original State. It is a primary method which is used to update the user interface(UI) in response to event handlers and server responses. Example
import React, { Component } from 'react';
import PropTypes from 'prop-types';
class App extends React.Component {
constructor() {
super();
this.state = {
msg: "Welcome to JavaTpoint"
};
this.updateSetState = this.updateSetState.bind(this);
}
updateSetState() {
this.setState({
msg:"Its a best ReactJS tutorial"
});
}
render() {
return (
<div>
<h1>{this.state.msg}</h1>
<button onClick = {this.updateSetState}>SET STATE</button>
</div>
);
}
}
export default App;
What is arrow function in React? How is it used?
The Arrow function is the new feature of the ES6 standard. If you need to use arrow functions, it is not necessary to bind any event to ‘this.’ Here, the scope of ‘this’ is global and not limited to any calling function. So If you are using Arrow Function, there is no need to bind ‘this’ inside the constructor. It is also called ‘fat arrow ‘(=>) functions.
//General way
render() {
return(
<MyInput onChange={this.handleChange.bind(this) } />
);
}
//With Arrow Function
render() {
return(
<MyInput onChange={ (e) => this.handleOnChange(e) } />
);
}
23) What is an event in React?
An event is an action which triggers as a result of the user action or system generated event like a mouse click, loading of a web page, pressing a key, window resizes, etc. In React, the event handling system is very similar to handling events in DOM elements. The React event handling system is known as Synthetic Event, which is a cross-browser wrapper of the browser’s native event.
Handling events with React have some syntactical differences, which are:
React events are named as camelCase instead of lowercase.
With JSX, a function is passed as the event handler instead of a string.
For More Information, Click here.
24) How do you create an event in React?
We can create an event as follows.
class Display extends React.Component({
show(msgEvent) {
// code
},
render() {
// Here, we render the div with an onClick prop
return (
<div onClick={this.show}>Click Me</div>
);
}
});
Example
import React, { Component } from 'react';
class App extends React.Component {
constructor(props) {
super(props);
this.state = {
companyName: ''
};
}
changeText(event) {
this.setState({
companyName: event.target.value
});
}
render() {
return (
<div>
<h2>Simple Event Example</h2>
<label htmlFor="name">Enter company name: </label>
<input type="text" id="companyName" onChange={this.changeText.bind(this)}/>
<h4>You entered: { this.state.companyName }</h4>
</div>
);
}
}
export default App;
function ActionLink() {
function handleClick(e) {
e.preventDefault();
console.log(‘You had clicked a Link.’);
}
return (
<a href=”#” onClick={handleClick}>
Click_Me
</a>
);
}
Lists are used to display data in an ordered format. In React, Lists can be created in a similar way as we create it in JavaScript. We can traverse the elements of the list using the map() function. Example
import React from 'react';
import ReactDOM from 'react-dom';
function NameList(props) {
const myLists = props.myLists;
const listItems = myLists.map((myList) =>
<li>{myList}</li>
);
return (
<div>
<h2>Rendering Lists inside component</h2>
<ul>{listItems}</ul>
</div>
);
}
const myLists = ['Peter', 'Sachin', 'Kevin', 'Dhoni', 'Alisa'];
ReactDOM.render(
<NameList myLists={myLists} />,
document.getElementById('app')
);
export default App;
28) What is the significance of keys in React?
A key is a unique identifier. In React, it is used to identify which items have changed, updated, or deleted from the Lists. It is useful when we dynamically created components or when the users alter the lists. It also helps to determine which components in a collection needs to be re-rendered instead of re-rendering the entire set of components every time. It increases application performance.
29) How are forms created in React?
Forms allow the users to interact with the application as well as gather information from the users. Forms can perform many tasks such as user authentication, adding user, searching, filtering, etc. A form can contain text fields, buttons, checkbox, radio button, etc.
React offers a stateful, reactive approach to build a form. The forms in React are similar to HTML forms. But in React, the state property of the component is only updated via setState(), and a JavaScript function handles their submission. This function has full access to the data which is entered by the user into a form.
import React, { Component } from 'react';
class App extends React.Component {
constructor(props) {
super(props);
this.state = {value: ''};
this.handleChange = this.handleChange.bind(this);
this.handleSubmit = this.handleSubmit.bind(this);
}
handleChange(event) {
this.setState({value: event.target.value});
}
handleSubmit(event) {
alert('You have submitted the input successfully: ' + this.state.value);
event.preventDefault();
}
render() {
return (
<form onSubmit={this.handleSubmit}>
<h1>Controlled Form Example</h1>
<label>
Name:
<input type="text" value={this.state.value} onChange={this.handleChange} />
</label>
<input type="submit" value="Submit" />
</form>
);
}
}
export default App;
30) What are the different phases of React component’s lifecycle?
The different phases of React component’s lifecycle are:
Initial Phase: It is the birth phase of the React lifecycle when the component starts its journey on a way to the DOM. In this phase, a component contains the default Props and initial State. These default properties are done in the constructor of a component.
Mounting Phase: In this phase, the instance of a component is created and added into the DOM.
Updating Phase: It is the next phase of the React lifecycle. In this phase, we get new Props and change State. This phase can potentially update and re-render only when a prop or state change occurs. The main aim of this phase is to ensure that the component is displaying the latest version of itself. This phase repeats again and again.
Unmounting Phase: It is the final phase of the React lifecycle, where the component instance is destroyed and unmounted(removed) from the DOM.
31) explain the lifecycle methods of React components in detail.
The important React lifecycle methods are:
getInitialState(): It is used to specify the default value of this.state. It is executed before the creation of the component.
componentWillMount(): It is executed before a component gets rendered into the DOM.
componentDidMount(): It is executed when the component gets rendered and placed on the DOM. Now, you can do any DOM querying operations.
componentWillReceiveProps(): It is invoked when a component receives new props from the parent class and before another render is called. If you want to update the State in response to prop changes, you should compare this.props and nextProps to perform State transition by using this.setState() method.
shouldComponentUpdate(): It is invoked when a component decides any changes/updation to the DOM and returns true or false value based on certain conditions. If this method returns true, the component will update. Otherwise, the component will skip the updating.
componentWillUpdate(): It is invoked before rendering takes place in the DOM. Here, you can’t change the component State by invoking this.setState() method. It will not be called, if shouldComponentUpdate() returns false.
componentDidUpdate(): It is invoked immediately after rendering takes place. In this method, you can put any code inside this which you want to execute once the updating occurs.
componentWillUnmount(): It is invoked immediately before a component is destroyed and unmounted permanently. It is used to clear up the memory spaces such as invalidating timers, event listener, canceling network requests, or cleaning up DOM elements. If a component instance is unmounted, you cannot mount it again.
32) What are Pure Components?
Pure components introduced in React 15.3 version. The React.Component and React.PureComponent differ in the shouldComponentUpdate() React lifecycle method. This method decides the re-rendering of the component by returning a boolean value (true or false). In React.Component, shouldComponentUpdate() method returns true by default. But in React.PureComponent, it compares the changes in state or props to re-render the component. The pure component enhances the simplicity of the code and performance of the application.
33) What are Higher Order Components(HOC)?
In React, Higher Order Component is an advanced technique for reusing component logic. It is a function that takes a component and returns a new component. In other words, it is a function which accepts another function as an argument. According to the official website, it is not the feature(part) in React API, but a pattern that emerges from React’s compositional nature.
34) What can you do with HOC?
You can do many tasks with HOC, some of them are given below:
- Code Reusability
- Props manipulation
- State manipulation
- Render highjacking
How to write comments in React?
In React, we can write comments as we write comments in JavaScript. It can be in two ways:
1. Single Line Comments: We can write comments as /* Block Comments */ with curly braces:
{/* Single Line comment */}
2. Multiline Comments: If we want to comment more that one line, we can do this as
{ /*
Multi
line
comment
*/ }
37) Why is it necessary to start component names with a capital letter?
In React, it is necessary to start component names with a capital letter. If we start the component name with lower case, it will throw an error as an unrecognized tag. It is because, in JSX, lower case tag names are considered as HTML tags.
38) What are fragments?
In was introduced in React 16.2 version. In React, Fragments are used for components to return multiple elements. It allows you to group a list of multiple children without adding an extra node to the DOM.
Example
render() {
return (
<React.Fragment>
<ChildA />
<ChildB />
<ChildC />
</React.Fragment>
)
}
There is also a shorthand syntax exists for declaring Fragments, but it's not supported in many tools:
render() {
return (
<>
<ChildA />
<ChildB />
<ChildC />
</>
)
}
39) Why are fragments better than container divs?
Fragments are faster and consume less memory because it did not create an extra DOM node.
Some CSS styling like CSS Grid and Flexbox have a special parent-child relationship and add <div> tags in the middle, which makes it hard to keep the desired layout.
The DOM Inspector is less cluttered.
40) How to apply validation on props in React?
Props validation is a tool which helps the developers to avoid future bugs and problems. It makes your code more readable. React components used special property PropTypes that help you to catch bugs by validating data types of values passed through props, although it is not necessary to define components with propTypes.
We can apply validation on props using App.propTypes in React component. When some of the props are passed with an invalid type, you will get the warnings on JavaScript console. After specifying the validation patterns, you need to set the App.defaultProps.
class App extends React.Component {
render() {}
}
Component.propTypes = { /*Definition */};
41) What is create-react-app?
Create React App is a tool introduced by Facebook to build React applications. It provides you to create single-page React applications. The create-react-app are preconfigured, which saves you from time-consuming setup and configuration like Webpack or Babel. You need to run a single command to start the React project, which is given below.
$ npx create-react-app my-app
This command includes everything which we need to build a React app. Some of them are given below:
- It includes React, JSX, ES6, and Flow syntax support.
- It includes Autoprefixed CSS, so you don’t need -webkit- or other prefixes.
- It includes a fast, interactive unit test runner with built-in support for coverage reporting.
- It includes a live development server that warns about common mistakes.
- It includes a build script to bundle JS, CSS, and images for production, with hashes and source maps.
42) How can you create a component in React?
There are two possible ways to create a component in React:
Function Components: This is the simplest way to create a component in React. These are the pure JavaScript functions that accept props object as the first parameter and return React elements:
function Greeting({ message }) {
return <h1>{`Hello, ${message}`}</h1>
}
Class Components: The class components method facilitates you to use ES6 class to define a component. The above function component can be written as:
class Greeting extends React.Component {
render() {
return <h1>{`Hello, ${this.props.message}`}</h1>
}
}
43) When do we prefer to use a class component over a function component?
If a component needs state or lifecycle methods, we should use the class component; otherwise, use the function component. However, after React 16.8, with the addition of Hooks, you could use state, lifecycle methods, and other features that were only available in the class component right in your function component.
44) Is it possible for a web browser to read JSX directly?
Web browsers can’t read JSX directly. This is because the web browsers are built to read the regular JS objects only, and JSX is not a regular JavaScript object.
If you want a web browser to read a JSX file, you must transform the files into a regular JavaScript object. For this purpose, Babel is used.
45) What do you understand by the state in React?
In react, the state of a component is an object that holds some information that may change over the component’s lifetime. It would be best to try to make your state as simple as possible and minimize the number of stateful components.
Let’s see how to create a user component with message state:
class User extends React.Component {
constructor(props) {
super(props)
this.state = {
message: 'Welcome to React world'
}
}
render() {
return (
<div>
<h1>{this.state.message}</h1>
</div>
)
}
}
The state is very similar to props, but it is private and fully controlled by the component. i.e., It is not accessible to any other component till the owner component decides to pass it.
46) What are the main changes that appear in React’s ES6 syntax compared to ES5 syntax?/How different is React’s ES6 syntax compared to ES5?
Following are the most visible syntax we can see while comparing ES6 and ES5:
require vs import
Syntax in ES5:
var React = require(‘react’);
Syntax in ES6:
import React from 'react';
export vs exports
Syntax in ES5:
module.exports = Component;
Syntax in ES6:
export default Component;
component and function
Syntax in ES5:
var MyComponent = React.createClass({
render: function() {
return
<h3>Hello JavaTpoint!</h3>
;
}
});
Syntax in ES6:
class MyComponent extends React.Component {
render() {
return
<h3>Hello JavaTpoint!</h3>
;
}
}
props
Syntax in ES5:
var App = React.createClass({
propTypes: { name: React.PropTypes.string },
render: function() {
return
<h3>Hello, {this.props.name}!</h3>
;
}
});
Syntax in ES6:
class App extends React.Component {
render() {
return
<h3>Hello, {this.props.name}!</h3>
;
}
}
state
Syntax in ES5:
var App = React.createClass({
getInitialState: function() {
return { name: 'world' };
},
render: function() {
return
<h3>Hello, {this.state.name}!</h3>
;
}
});
Syntax in ES6:
class App extends React.Component {
constructor() {
super();
this.state = { name: 'world' };
}
render() {
return
<h3>Hello, {this.state.name}!</h3>
;
}
}
47) What do you understand by props in React?
In React, the props are inputs to components. They are single values or objects containing a set of values passed to components on creation using a naming convention similar to HTML-tag attributes. They are data passed down from a parent component to a child component.
The main purpose of props in React is to provide the following component functionality:
- Pass custom data to your component.
- Trigger state changes.
- Use via this.props.reactProp inside component’s render() method.
For example, let us create an element with reactProp property:
<Element reactProp={‘1’} />
This reactProp name becomes a property attached to React’s native props object, which already exists on all React library components.
props.reactProp
48) What do you understand by refs in React?
Refs is the shorthand used for references in React. It is an attribute which helps to store a reference to particular DOM nodes or React elements. It provides a way to access React DOM nodes or React elements and how to interact with it. It is used when we want to change the value of a child component, without making the use of props.
49) How to create refs?
Refs can be created by using React.createRef() and attached to React elements via the ref attribute. It is commonly assigned to an instance property when a component is created, and then can be referenced throughout the component.
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.callRef = React.createRef();
}
render() {
return <div ref={this.callRef} />;
}
}
50) What are Forward Refs?
Ref forwarding is a feature which is used for passing a ref through a component to one of its child components. It can be performed by making use of the React.forwardRef() method. It is particularly useful with higher-order components and specially used in reusable component libraries.
Example
import React, { Component } from 'react';
import { render } from 'react-dom';
const TextInput = React.forwardRef((props, ref) => (
<input type="text" placeholder="Hello World" ref={ref} />
));
const inputRef = React.createRef();
class CustomTextInput extends React.Component {
handleSubmit = e => {
e.preventDefault();
console.log(inputRef.current.value);
};
render() {
return (
<div>
<form onSubmit={e => this.handleSubmit(e)}>
<TextInput ref={inputRef} />
<button>Submit</button>
</form>
</div>
);
}
}
export default App;
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