React JS interview questions for freshers/React JS Interview Questions and Answers for Freshers & Experienced

When to use a Class Component over a Function Component?

If the component needs state or lifecycle methods then use class components. Otherwise use functional components. From React 16.8 with the addition of Hooks, you could use state, lifecycle methods available in class components right in your function component.

Example –

import React from ‘react’’
class Class_component extends React.Component {
render() {
return <h1>{`Hello, ${this.props.message}`}</h1>
}
}

How to debug React JS?

One of the cyclic ways to debug React code on the web using console.log(),console.warn, console.error, and similar statements. you will do console.log() ,You will then see the result in the browser inspector.

What are controlled and uncontrolled components in React?

This relates to stateful DOM components (form elements) and the difference:
* A Controlled Component is one that takes its current value through props and notifies changes through callbacks like onChange. A parent component “controls” it by handling the callback and managing its own state and passing the new values as props to the controlled component. You could also call this a “dumb component”.

* A Uncontrolled Component is one that stores its own state internally, and you query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML.
In most (or all) cases we should use controlled components.

What is render() in React? And explain its purpose?

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.

How to declare a variable in React jS?

setState() actions are asynchronous and are batched for performance gains. This is explained in documentation as below.

setState() does not immediately mutate this.state but creates a pending state transition. Accessing this.state after calling this method can potentially return the existing value. There is no guarantee of synchronous operation of calls to setState and calls may be batched for performance gains.

This is because setState alters the state and causes rerendering. This can be an expensive operation and making it synchronous might leave the browser unresponsive. Thus the setState calls are asynchronous as well as batched for better UI experience and performance.

How to create an event in React?

An event can be created in React using the following:

class Display extends React.Component({

show(msgEvent) {

},

render() {

return (

<div onClick={this.show}>Click Me</div>

);

}

});

How to check the React jS version?

When you create a react app that time generates a package.json file .In package.json check dependencies: {} section.
Second way is

$create-react-app –version

How to import jS files in react?

First you export the js file that means every jS file in reactjS is a component, export this file App.js.

Example –

import React from ‘react’;
Class ExportFile ( () = >{
render(
return(
<h1> Export file</h1>
)
)}
Export default ExportFile

How to install bootstrap in React JS?

The best way to install react- bootstrap via NPM (Node Package Manager).

The command for installing react-bootstrap in react js is below: –

$ npm install react-bootstrap bootstrap

After successfully installation import the bootstrap library

Example –

import Button from ‘react-bootstrap/Button’;

import { Button } from ‘react-bootstrap’; //import bootsrap button

How to create a website in React JS?

STEP 1: Create a react app

$ npx create-react-app App_name

STEP 2: Install the dependencies

STEP 3: Create components

STEP4: deploy the application

What is babel in React JS?

* Babel is a toolchain that is mainly used to convert ECMAScript code into a backwards compatible version of JavaScript.
* Babel has support for the latest version of JavaScript through syntax transformers.
* Babel can convert JSX syntax.

How to connect MongoDB with React JS?

React is a front end framework you will not have access to things on your backend such as methods like db.collection.insert(). You will in turn have a separation of front end code and back end code. They will talk to each other through HTTP requests (GET, POST, PUT, DELETE).

Example –

var mongoose = require('mongoose');
var Message = require('../models/message');

app.post('/message', (request, response) => {
var newMessage = new Message(request.body);
newMessage.save((error, doc) => {
if (error) {
response.send(error);
} else {
response.send(doc);
}
});
});

What is Flux and Redux?

Flux – Flux is a Javascript pattern for UI which runs on a unidirectional data flow. It is useful when your project has dynamic data and you need to keep the data updated in an effective manner.

Redux – Redux is a predictable state container for JavaScript apps based on the Flux design pattern. Redux can be used together with React, or with any other view library. It is tiny (about 2kB) and has no dependencies. React Redux is the official React UI bindings layer for Redux. It lets your React components read data from a Redux store, and dispatch actions to the store to update state.

How to setup React JS?

When you create react app using create-react-app App_name that time generate some folders like as: build/, node_modules/, public/, src/, .gitignore/, README.md, package-lock.json,Package.json.

In these folders, some folders or files are not useable. Such as, .gitignore, package-lock.json and in a public folder, this file is not usable: favicon.ico, logo192.png, manifest.json, logo512.png, robots.txt. Replace your files with this file and use.

What is a router in React JS?

React Router is a powerful routing library built on top of React that helps you add new screens and flows to your application incredibly quickly, all while keeping the URL in sync with what’s being displayed on the page.

Explain the different phases of the ReactJS component lifecycle?

A child class constructor cannot make use of this reference until the super() method is invoked. The main reason of using super() constructor is to grant access to child constructors to use this.props reference.

Example –

class Demo extends React.Component {
constructor(props) {
super(props)

console.log(this.props) // prints { name: 'John', age: 42 }
}
}

What is context?

Context provides a way to pass data via component tree without passing it to props manually at every level.

For example, authenticated user, locale preference, UI theme need to be accessed in the application by many components.

Const {Provider, Consumer} = React.createContext (defaultValue)

What is reconciliation?

Reconciliation is the process through which React updates the DOM. When a component’s state changes, React has to decide whether it is necessary to update the DOM. It does this by creating a virtual DOM and comparing it with the current DOM. In this context, the virtual DOM will contain the new state of the component.

Explain strict mode in React.

Strict mode is a tool for pointing out potential problems in an application. Like Fragment , Strict mode does not render any visible UI. It activates additional checks and warnings for its descendants.

How Virtual-DOM is more efficient than Dirty checking?

In React, each of our components have a state. This state is like an observable. Essentially, React knows when to re-render the scene because it is able to observe when this data changes. Dirty checking is slower than observables because we must poll the data at a regular interval and check all of the values in the data structure recursively. By comparison, setting a value on the state will signal to a listener that some state has changed, so React can simply listen for change events on the state and queue up re-rendering.

The virtual DOM is used for efficient re-rendering of the DOM. This isn’t really related to dirty checking your data. We could re-render using a virtual DOM with or without dirty checking. In fact, the diff algorithm is a dirty checker itself.

We aim to re-render the virtual tree only when the state changes. So using an observable to check if the state has changed is an efficient way to prevent unnecessary re-renders, which would cause lots of unnecessary tree diffs. If nothing has changed, we do nothing.

What are pure components?

Pure components are the components whose output is determined only by it’s input that is props. Pure component is similar to component except that it handles the shouldComponentUpdate method.

What is Redux?

Redux is an open-source, JavaScript library used to manage the application state. React uses Redux to build the user interface. It is a predictable state container for JavaScript applications and is used for the entire application’s state management.

What is a higher-order component in React?

A higher-order component acts as a container for other components. This helps to keep components simple and enables re-usability. They are generally used when multiple components have to use a common logic.

Why can’t browsers read JSX?

Browsers can read JavaScript objects but JSX in not a JavaScript object. Hence, to allow a browser to read JSX, first, we need to modify JSX file into a JavaScript object using JSX transformers and then pass it to the browser.

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.

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.

What are Pure Components in React?

Pure components are those components that render the same output for the same state and props. They are the simplest and fastest components that can be written. They can replace any component which only has a render(). Thus, a function is said to be pure if:

1. Its return value is determined only by its input values
2. Its return value is always the same for the same input values

What is the difference between element and component?

Element – An element is an object describing what you would like to project on the screen in terms of the DOM nodes or other components. Elements can contain other elements in their props. Once a component is created , it’s never mutated.

Example –

<button class=”green”> </button>
Component – A component is a function or class that accepts an input and returns a React element. It has to keep references to its DOM nodes and to the instances of the child components.

Example –

const LogIn = () => (
<div>
<p> Login </p>
<button> Continue </button>
<button color = “blue”> Cancel </button>
</div>
);

What is the purpose of render() in React?

The render() function in React displays the specified HTML code inside the specified HTML element. It is mandatory for each React component to have a render(). This function takes 2 arguments, namely HTML code and an HTML element, and returns a single element which is the representation of the native DOM component.

If more than one HTML element is to be rendered, then they must be grouped inside one enclosing tag, such as <form>, <group>, and <div>.

What are States in React?

States are the source of data. They are objects which determine components rendering and behavior. While props are immutable, States are mutable and can change over time. States create dynamic and interactive components and are accessed by using this.state(). While using the State component, you will set a component’s default state, access the current state, and update the state.

What is Props?

Short for properties, Props are arguments passed into React components. They are like function arguments in JavaScript. Props accept arbitrary inputs and return React elements detailing what should be displayed on the screen. Props maintain a unidirectional data flow and are only passed down from the parent component to the child components throughout the application. A child component can never send a prop back to the parent component. They are immutable.

What are React Hooks?

Hooks enable you to use state and other React features without writing a class. These in-built functions developers to use state and lifecycle methods inside functional components. They do not work inside classes. By using Hooks, you can completely avoid the use of lifecycle methods, like componentDidMount, componentDidUpdate, and componentWillUnmount.

What are Higher Order Components (HOC)?

Higher Order Components are an advanced way of reusing the component logic. They are a pattern derived from React’s compositional nature and are not considered a part of the React API. HOC are functions that take a component and return a new component. HOCs are pure components as they can accept any dynamically provided child component, however, they will not copy or modify any behavior from their input components.

Explain the new lifecycle methods in React 16.3?

There are two new lifecycle methods introduced in React 16.3. Their main task is replacing the old error-prone lifecycle methods like componentWillUpdate and componentWillReceiveProps. You can still use these old lifecycle methods in your project but only till React 17 is released. These old lifecycle methods are now called - UNSAFE_componentWillUpdate and UNSAFE_componentWillReceiveProps.

static getDerivedStateFromProps(nextProps, prevState)
This lifecycle method is invoked before calling render(), both on the initial mount and subsequent mounts. It’s main job is replacing componentWillReceiveProps which is now called UNSAFE_componentWillReceiveProps.

getSnapshotBeforeUpdate(prevProps, prevState)
This lifecycle method is called right before the changes from VDOM are to be committed to the Real DOM. It enables your component to capture some information from the DOM like mouse position before it is changed. The returned value by this lifecycle will be passed as a parameter to componentDidUpdate().

So, the new order of mounting is -

1. constructor()
2. static getDerivedStateFromProps()
3. render()
4. componentDidMount()

The order of update caused by changes in any props or state is -

1. static getDerivedStateFromProps()
2. shouldComponentUpdate()
3. render()
4. getSnapshotBeforeUpdate()
5. componentDidUpdate()

What are refs in React?

Refs are a function in React that acts as a way to reference a DOM element or a class component from within a parent component. It enables you to read and modify the value of a child component, without making use of props. Refs can be helpful when using third party libraries as well in animations.

What is the significance of Keys in React?

Keys are used in React to identify unique VDOM Elements with their corresponding data driving the UI and help React identify which items have changed, are added, or are removed. Keys should be a unique number or string.

Explain the lifecycle methods of ReactJS?

The lifestyle methods of ReactJS are:

1. componentWillMount: This method is used for App-level configuration in the root component. It is executed before rendering

2. componentDidMount: This method is executed after first rendering only on the client-side. In this, all AJAX requests, DOM or state updates, and more take place.
componentWillReceiveProps is invoked when the props are updated and before the next render is called.

3. shouldComponentUpdate: It should return a true or false value and will determine if the component will be updated or not. This is set to true by default. If you are sure that a component doesn’t need to render after state or props are updated, then you can return a false value.

4. componentWillUpdate: It is invoked just before re-rendering the component.

5. componentDidUpdate: It is invoked just after rendering and is used to update the DOM in response to prop or state changes.

6. componentWillUnmount: It is called after the component is unmounted from the DOM.

What is React.createClass?

Since JavaScript didn’t have classes, React has its own class system. React.createClass allows you to generate component classes. Thus, the component class uses a class system implemented by React.

With ES6, React enables you to implement component classes that use ES6 JavaScript classes. While the way of doing is different, the end result would be the same.

What are controlled components?

In HTML, form elements such as <input>, <textarea>, and <select> typically maintain their own state and update it based on user input. When a user submits a form the values from the aforementioned elements are sent with the form. With React it works differently. The component containing the form will keep track of the value of the input in it's state and will re-render the component each time the callback function e.g. onChange is fired as the state will be updated. A form element whose value is controlled by React in this way is called a "controlled component".

With a controlled component, every state mutation will have an associated handler function. This makes it straightforward to modify or validate user input.

What are the differences between a class component and functional component?

Class components allows us to use additional features such as local state and lifecycle hooks. Also, to enable our component to have direct access to our store and thus holds state.

When our component just receives props and renders them to the page, this is a ‘stateless component’, for which a pure function can be used. These are also called dumb components or presentational components.

What is ReactDOM and what is the difference between ReactDOM and React?

* Prior to v0.14, all ReactDOM functionality was part of React. But later, React and ReactDOM were split into two different libraries.

* As the name implies, ReactDOM is the glue between React and the DOM. Often, we will only use it for one single thing: mounting with ReactDOM. Another useful feature of ReactDOM is ReactDOM.findDOMNode() which we can use to gain direct access to a DOM element.

* For everything else, there’s React. We use React to define and create our elements, for lifecycle hooks, etc. i.e. the guts of a React application.

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.

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

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.

What is the virtual DOM?

DOM stands for Document Object Model. The DOM represents an HTML document with a logical tree structure. Each branch of the tree ends in a node, and each node contains objects.
React keeps a lightweight representation of the real DOM in the memory, and that is known as the virtual DOM. When the state of an object changes, the virtual DOM changes only that object in the real DOM, rather than updating all the objects.

Can web browsers read JSX directly?

* Web browsers cannot read JSX directly. This is because they are built to only read regular JS objects and JSX is not a regular JavaScript object .

* For a web browser to read a JSX file, the file needs to be transformed into a regular JavaScript object. For this, we use Babel.

What is JSX?

JSX is a syntax extension of JavaScript. It is used with React to describe what the user interface should look like. By using JSX, we can write HTML structures in the same file that contains JavaScript code.

What are the advantages of React?

The advantages of React are:

* It ensures faster rendering.
* It is SEO-friendly.
* React combines the speed of JavaScript and uses a new approach to render web
pages to make them dynamic and responsive.
* It can be used for the development of both web and mobile apps.
* JSX makes components/blocks code readable.
* It is easy to integrate with other frameworks, such as Angular and Meteor.

What are the features of React?

The features of React are:

* ReactJS follows a unidirectional data flow.
* It uses virtual DOM instead of the real DOM.
* React has a fully component-based architecture.
* It uses server-side rendering.

What is React?

React is an open-source JavaScript library that is used to develop complex and interactive web and mobile UI. It is used for building web browser apps, creating dynamic libraries, and building UI’s that are perfectly equipped to render large datasets. React follows the component-based approach to create reusable and complex UIs from small and isolated pieces of code called components.

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