React js Interview Questions and Answers Series – Part 1

In this article, I have shared the Top 20 React js Interview Questions and Answers for beginners. This is part 1 of the react.js interview question and answers series. I will post other parts as well where I will increase the difficulty level.

React js Interview Questions and Answers


React js Interview Question and Answers Series – Part 1


1. What is React.js?

React.js is a JavaScript library that is used to build responsive user interfaces. It is designed to be declarative, efficient, and flexible, and the same has been developed and maintained by Facebook.


2. What are the key features of React.js?

Below are some of the key features of React.js


a) Declarative Components: React allows developers to create reusable UI components using declarative syntax. This makes it easier to understand and work with the code, and also makes it easier to reason about the behavior of the application.


b)Virtual DOM: React uses a virtual DOM to optimize updates to the actual DOM. When a component’s state changes, React creates a new virtual DOM tree and compares it to the previous tree. It then makes the minimal number of changes necessary to the actual DOM, which helps to improve the performance of the page.


c) React JSX: React uses JSX, a syntax extension for JavaScript, to describe the structure of UI components. JSX looks similar to HTML, but it is actually a declarative syntax for creating and composing UI components in JavaScript.


d)One-Way Data Flow: React follows a one-way data flow pattern, where the parent component passes data down to its children through props (short form for “properties”). This helps to enforce a unidirectional data flow, which can make it easier to reason about the state of the application.


e) React Native: React Native is a framework for building native mobile apps using React. It allows to build of iOS and Android apps using the same declarative component-based structure as React, but with native components instead of web components.

3. What is the difference between props and states in React.js?

The props in React.js is a piece of data that is passed to a component from its parent. Where the state, on the other hand, is a piece of data that is managed within the component itself and can change over time. Props are immutable, while the state can be changed within the component using the setState() method.

4. How does virtual DOM works in React.js?

The virtual DOM is an abstract representation of the actual DOM structure. It allows React.js to update the DOM more efficiently by minimizing the number of DOM mutations that need to be made.

When a React component’s state or props change or update, React.js re-renders the component and updates the virtual DOM. It then compares both the virtual DOM and the actual DOM and only makes the necessary changes to the actual DOM, hence it improving the application performance.


5. How do you create a new React.js component?

To create a new React.js component, you can define a function or a class that returns a JSX element. For example:

function firstComponent() {
  return <div>Hello, World!</div>;

Class Component

class firstComponent extends React.Component {
  render() {
    return <div>Hello, World!</div>;

6. How do you pass data between components in React.js?

Below are a few ways to pass data between components in React.js-

a) Passing data as props to child components.
b) Using a state management library such as Redux.
c) Using the React.js context API.
d) Using the React.js render props pattern.

7. What is a higher-order component in React.js?

The higher-order component is a method that takes a component as an argument and returns a new component. it can be used to add additional functionality to a component, such as adding a state or props to it.

8. How do you handle events in React.js with examples?

React.js events are named using camelCase.To handle events in React.js, you can use the onClick, onChange,onSubmit, or other event handler attributes in your JSX code. For example

<button onClick={() => console.log('Button clicked!')}>Click me</button>

Example 2 –

function Form() {
function handleSubmit(e) {
console.log('You clicked submit.');

return (
<form onSubmit={handleSubmit}>
<button type="submit">Submit</button>


9. How do you perform conditional rendering in React.js?

The conditional rendering in React.js can be performed using the if statement or a ternary operator within your JSX code. For example:

{isLoading ? Loading... :contenttoLoad}

10. How do you use the React.js lifecycle methods?

The React.js lifecycle methods are called at different stages of a component’s lifecycle, such as when it is mounted, updated, or unmounted. Some common lifecycle methods include componentDidMount(), shouldComponentUpdate(), and componentWillUnmount().

There are a few other methods as well but the lifecycle has the above 3 stages.


11. How do you debug a React.js application?

The best way to debug the React.js application use a developer tools browser extension or the console in your browser’s developer tools. You can also use a debugger statement in your code where you want to pause execution and inspect the state of your application.


12. What is the purpose of the shouldComponentUpdate lifecycle method in React.js?

The React shouldComponentUpdate() lifecycle method allows you to control when a component should re-render. By default, a component will re-render every time its state or props change.

However, if you want to optimize performance, you can use shouldComponentUpdate() to prevent unnecessary re-renders by returning false if the component does not need to update.


13. What is the difference between React.js and React Native?

As you know React.js and React Native are both developed and maintained by Facebook and are based on the same principles of reactive programming and the virtual DOM. However, React.js is used to build web applications, while React Native is used to building native mobile applications for iOS and Android users.


14. What is the difference between functional and class-based components in React.js?

In React.js code, you can create components using either a function or a class. Function-based components are also known as functional components or stateless components. They are simpler and easier to write, but they do not have access to state or lifecycle methods while the Class-based components, on the other hand, are also known as stateful components. They can have state and lifecycle methods, but they are more verbose and can be a little bit harder to understand.


15. What is the difference between the stateful and the stateless components in React.js?

The stateful component in React.js is a component that has a state object and can update its state using the setState() method while the stateless component, on the other hand, is a component that does not have a state object and does not have the capability to update its own state. Stateless components are typically used for presentational purposes and receive all of their data as props from their parent component.

An example of a stateful component is the class component where you can update the state using the setState() method and the functional component is referred to as a stateless component.


16. What is the difference between the React.js createElement and JSX syntax?

JSX is a syntax extension for JavaScript that allows you to write HTML-like code within your JavaScript code. It is used to build the UI in React.js applications and is converted to JavaScript by the React.js compiler. The React.createElement(), on the other hand, is used for creating React elements and It is not as user-friendly as JSX.


17. What are the benefits of using JSX in React.js?

Below are some key benefits of using JSX in React.js –
a) It makes the code easier to read, understand and maintain, especially for developers who are familiar with HTML.
b) It allows you to write concise, declarative code for building UI components.
c) Less line of tags-based code which is faster than normal javascript.


18. What is the purpose of the componentDidMount lifecycle method in React?

The componentDidMount() lifecycle method is called after a component has been mounted. It is a good place to perform any operation or API calls that should happen after the component has been rendered.


19. What is the purpose of the render() method in a React component?

The render() method is a required part of React lifecycle methods and It returns the JSX that should be rendered for the component. The render method is called whenever the component’s state or props changed.


20. How do you optimize a React application for performance?

To optimize the React.js application performance, you can follow below best practices such as-

a) By using the shouldComponentUpdate() lifecycle method to prevent unnecessary re-renders of the page.
b) By using React.memo higher-order components to memoize functional components to avoid unnecessary rendering.
c) By using React.lazy() function and the Suspense component to lazy load components.
d) By using React.usecallback hook to optimizing the rendering of expensive components.


I hope these top 20 react js interview questions and answers will help in interview preparation. Good Luck!