In this article, you will learn ReactJS interview questions and answers that are most frequently asked in interviews. Before proceeding to learn ReactJS interview questions and answers, first learn the complete ReactJS Tutorial.
ReactJS Interview Questions and Answers
ReactJS is an open-source JavaScript library that is used for building user interfaces in a declarative and efficient way. It is a component-based front-end library responsible only for the view layer of an MVC (Model View Controller) architecture. React is used to create modular user interfaces and it promotes the development of reusable UI components that display dynamic data.
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Let’s discuss some common questions that you should prepare for the interviews. These questions will be helpful in clearing the interviews specially for the frontend development or full stack development role.
This set contains the basic questions asked in the interview.
1. What is ReactJS?
ReactJS is a JavaScript library used to build reusable components for the view layer in MVC architecture. It is highly efficient and uses a virtual DOM to render components. It works on the client side and is written in JSX.
2. Explain the MVC architecture?
The Model-View-Controller (MVC) framework is an architectural/design pattern that separates an application into three main logical components Model, View, and Controller. Each architectural component is built to handle specific development aspects of an application. It isolates the business, logic, and presentation layer from each other
3. Explain the building blocks of React?
The five main building blocks of React are:
- Components: These are reusable blocks of code that return HTML.
- JSX: It stands for JavaScript and XML and allows to write HTML in React.
- Props and State: props are like function parameters and State is similar to variables.
- Context: This allows data to be passed through components as props in a hierarchy.
- Virtual DOM: It is a lightweight copy of actual DOM which makes DOM manipulation easier.
4. Explain props and state in React with differences
Props are used to pass data from one component to another. The state is local data storage that is local to the component only and cannot be passed to other components.
|
PROPS |
STATE |
|---|---|
| The Data is passed from one component to another. | The Data is passed within the component only. |
| It is Immutable (cannot be modified). | It is Mutable ( can be modified). |
| Props can be used with state and functional components. | The state can be used only with the state components/class component (Before 16.0). |
| Props are read-only. | The state is both read and write. |
5. What is virtual DOM in React?
React uses Virtual DOM exists which is like a lightweight copy of the actual DOM(a virtual representation of the DOM). So for every object that exists in the original DOM, there is an object for that in React Virtual DOM. It is exactly the same, but it does not have the power to directly change the layout of the document. Manipulating DOM is slow, but manipulating Virtual DOM is fast as nothing gets drawn on the screen. So each time there is a change in the state of our application, the virtual DOM gets updated first instead of the real DOM.
6. What is JSX?
JSX is basically a syntax extension of regular JavaScript and is used to create React elements. These elements are then rendered to the React DOM. All the React components are written in JSX. To embed any JavaScript expression in a piece of code written in JSX we will have to wrap that expression in curly braces {}.
Example of JSX: The name written in curly braces { } signifies JSX
Javascript
const name = "Learner"; const element = <h1>Hello, { name }.Welcome to neveropen.< /h1>; |
7. What are components and its type in React?
A Component is one of the core building blocks of React. In other words, we can say that every application you will develop in React will be made up of pieces called components. Components make the task of building UIs much easier.
In React, we mainly have two types of components:
- Functional Components: Functional components are simply javascript functions. We can create a functional component in React by writing a javascript function.
- Class Components: The class components are a little more complex than the functional components. The functional components are not aware of the other components in your program whereas the class components can work with each other. We can pass data from one class component to another class component.
8. How do browsers read JSX?
In general, browsers are not capable of reading JSX and only have the capacity to read pure JavaScript. The web browsers read JSX with the help of a transpiler. Transpilers are used to convert JSX into JavaScript. The transpiler used is called Babel
9. Explain the steps to create a react application and print hello world?
To install react, first, make sure Node is installed on your computer. After installing Node. Open the terminal and type the following command.
npx create-react-app <<Application_Name>>
Navigate to the folder.
cd <<Application_Name>>
This is the first code of ReactJS Hello World!
Javascript
import React from 'react'; import './App.css'; function App() { return ( <div className="App"> Hello World ! </div> ); } export default App; |
Type the following command to run the application
npm start
10. How to create an event in React?
To create an event write the following code.
Javascript
function Component(){ doSomething(e){ e.preventDefault(); // Some more response to the event } return ( <button onEvent={doSomething}></button> ); } |
11. Explain the creation of a List in react?
Lists are very useful when it comes to developing the UI of any website. Lists are mainly used for displaying menus on a website, for example, the navbar menu. To create a list in React use the map method of array as follows.
Javascript
import React from 'react'; import ReactDOM from 'react-dom'; const numbers = [1,2,3,4,5]; const updatedNums = numbers.map((number)=>{ return <li>{number}</li>; }); ReactDOM.render( <ul> {updatedNums} </ul>, document.getElementById('root') ); |
12. What is a key in React?
A “key” is a special string attribute you need to include when creating lists of elements in React. Keys are used in React to identify which items in the list are changed, updated, or deleted. In other words, we can say that keys are used to give an identity to the elements in the lists.
13. How to write a comment in React?
There are two ways to write comments in React.
- Multi-line comment: We can write multi-line comments in React using the asterisk format /* */.
- Single line comment: We can write single comments in React using the double forward slash //.
14. Explain the difference between React and Angular?
Field |
React.js |
Angular |
|---|---|---|
| Used as |
React.js is a JavaScript library. As it indicates react js updates only the virtual DOM is present and the data flow is always in a single direction. |
Angular is a framework. Angular updates the Real DOM and the data flow is ensured in the architecture in both directions. |
| Architecture |
React.js is more simplified as it follows MVC ie., Model View Control. |
The architecture is complex as it follows MVVM models ie., Model View-ViewModel. |
| Scalability | It is highly scalable. | It is less scalable than React JS. |
| Data Binding | It supports Uni-directional data binding which is one-way data binding. | It supports Bi-directional data binding which is two data binding. |
| DOM | It has virtual DOM. | It has regular DOM. |
15. Explain the use of render method in React?
React renders HTML to the web page by using a function called render(). The purpose of the function is to display the specified HTML code inside the specified HTML element. In the render() method, we can read props and state and return our JSX code to the root component of our app.
16. What is state in React?
The state is an instance of React Component Class that can be defined as an object of a set of observable properties that control the behavior of the component. In other words, the State of a component is an object that holds some information that may change over the lifetime of the component.
17. Explain props in React?
React allows us to pass information to a Component using something called props (which stands for properties). Props are objects which can be used inside a component
We can access any props inside from the component’s class to which the props is passed. The props can be accessed as shown below:
this.props.propName;
18. What is higher-order component in React?
Higher-order components or HOC is the advanced method of reusing the component functionality logic. It simply takes the original component and returns the enhanced component. HOC are beneficial as they are easy to code and read. Also, helps to get rid of copying same logic in every component.
19. Explain the difference between functional and class component in React?
| Functional Components | Class Components |
|---|---|
| A functional component is just a plain JavaScript pure function that accepts props as an argument | A class component requires you to extend from React. Component and create a render function |
| No render method used | It must have the render() method returning JSX |
| Also known as Stateless components | Also known as Stateful components |
| React lifecycle methods (for example, componentDidMount) cannot be used in functional components. | React lifecycle methods can be used inside class components (for example, componentDidMount). |
| Constructors are not used. | Constructor is used as it needs to store state. |
20. Explain one way data binding in React?
ReactJS uses one-way data binding which can be Component to View or View to Component. It is also known as one-way data flow, which means the data has one, and only one way to be transferred to other parts of the application. In essence, this means child components are not able to update the data that is coming from the parent component. It is easy to debug and less prone to errors.
