Top 30+ React.js Interview Questions You Must Prepare In 2023

React.js has become one of the most popular JavaScript libraries for building user interfaces. With its growing popularity, more and more companies are looking for skilled React.js developers. If you’re looking to land a React.js job in 2023, it’s important to be prepared for the interview. To help you get started, here are the top 30+ React.js interview questions you must prepare for in 2023.

React.js Interview Questions With Answers

• What is React.js, and what are its advantages?

React.js is a popular JavaScript library used for building user interfaces. It was created by Facebook and is now maintained by both Facebook and a community of developers. React.js allows developers to create reusable UI components and manage the state of these components effectively. It uses a virtual DOM and can be used with other libraries or frameworks. Some of the advantages of React.js include its ability to improve application performance, its flexibility, and its large community of developers.

• What is the virtual DOM in React.js, and how does it work?

The virtual DOM in React.js is a lightweight copy of the actual DOM. It is a representation of the UI components that are rendered in the browser. When a change occurs in a React.js application, the virtual DOM is updated, and React calculates the difference between the previous and new virtual DOMs. Only the changes that need to be made are then applied to the actual DOM, making the rendering process more efficient.

• What is JSX, and how does it differ from HTML?

JSX is a syntax extension for JavaScript that allows developers to write HTML-like code in their JavaScript files. It allows for the easy creation of React elements and components by combining HTML and JavaScript syntax. JSX is not HTML but is similar in its structure and tags. The main difference between JSX and HTML is that JSX uses JavaScript expressions to create dynamic content.

• How do you create a React component, and what are the different types of components?

To create a React component, you can use either a function or a class. A function component is a simple JavaScript function that takes in props as an argument and returns a React element. A class component is a JavaScript class that extends React. Component class and includes a render method that returns a React element. There are also two types of components in React: Presentational components, which are concerned with how things look, and Container components, which are concerned with how things work.

• What is the difference between a stateful component and a stateless component?

A stateful component, also known as a container component, manages its own state and can update it using the setState() method. It can also pass its state down to child components as props. A stateless component, also known as a presentational component, does not manage any state and receives all of its data via props from its parent component. It is generally used for displaying static content.

• What is the significance of the constructor method in React?

The constructor method in React is used to initialize the state of a component, bind event handlers to the component, and set the initial values of the component’s props. It is called when the component is first created.

• What is a prop in React, and how is it used?

A prop in React is a short form for “property” and is used to pass data from one component to another. Props are passed as attributes to a component in JSX and can be accessed in the component’s code as an object.

• What is the difference between state and props in React?

State is used to manage the internal state of a component, while props are used to pass data from a parent component to a child component. State is managed within the component, while props are passed down from the parent component.

• How do you update the state of a component in React?

To update the state of a component in React, you can call the setState() method. This method takes an object as an argument that represents the new state of the component. The new state is then merged with the old state, and the component is re-rendered with the updated state.

• What is the significance of the componentDidMount method in React?

The componentDidMount method in React is a lifecycle method that is called after a component is mounted in the DOM. It is used to perform any necessary setup, such as fetching data from an API, setting up event listeners, or initializing a third-party library. This method is only called once during the lifecycle of a component.

• What is the difference between props and state in React?

Props and state are both used to manage data in a React component, but they serve different purposes. Props are used to pass data from a parent component to a child component, while state is used to manage the internal state of a component. Props are read-only and cannot be modified by the child component, while state can be updated by the component itself.

• What is the significance of the componentWillUnmount method in React?

The componentWillUnmount method in React is a lifecycle method that is called when a component is about to be removed from the DOM. It is used to perform any necessary cleanup, such as removing event listeners or canceling pending requests. This method is only called once during the lifecycle of a component.

• What is the difference between the state and props in React?

The state and props are both used to manage data in a React component, but they have different purposes. The state is used to manage the internal state of a component, while props are used to pass data from a parent component to a child component. The state is mutable, and changes to the state can trigger a re-render of the component, while props are read-only and cannot be modified by the child component.

• What is a Higher-Order Component (HOC) in React?

A Higher-Order Component (HOC) is a function that takes in a component as an argument and returns a new component with additional functionality. HOCs are used to add reusable logic or behavior to a component, such as adding authentication, caching, or error handling.

• What is Redux, and how is it used in React?

Redux is a state management library for JavaScript applications. It is often used in conjunction with React to manage the application state. Redux provides a centralized store that holds the state of the entire application and provides methods to update and access that state. React components can subscribe to the Redux store to receive updates when the state changes.

• What is the difference between React and Angular?

React and Angular are both popular JavaScript frameworks for building web applications, but they have different approaches and philosophies. React is a library, not a full-fledged framework, and focuses on building UI components. Angular, on the other hand, is a complete framework that includes everything needed to build a web application, including routing, state management, and testing. React is often used with other libraries and frameworks, while Angular is a more self-contained ecosystem.

• What is the purpose of the shouldComponentUpdate method in React?

The shouldComponentUpdate method in React is a lifecycle method that is called before a component is updated. It allows the component to determine whether a re-render is necessary based on the changes to its props or state. By default, React will always re-render a component when its props or state change, but the shouldComponentUpdate method allows developers to optimize performance by preventing unnecessary re-renders.

• What is the React context API, and how is it used?

The React context API is a feature introduced in React 16.3 that allows data to be passed down the component tree without having to pass props explicitly at every level. Context provides a way to share data between components that are not directly related. Context can be useful for providing theme data, user preferences, or localization data to multiple components.

• What is the purpose of the React Router library?

The React Router library is used for handling routing in React applications. It provides a way to define routes for different URLs and map those routes to specific components. This allows for dynamic content to be displayed based on the URL, such as displaying different views for different pages or handling authentication and authorization based on the route.

• What is React Native, and how is it used?

React Native is a framework for building native mobile applications using JavaScript and React. It allows developers to use the same codebase for both iOS and Android apps, reducing development time and cost. React Native provides a set of components and APIs that are optimized for mobile devices and can be used to create complex, interactive user interfaces.

• What is the purpose of the useReducer hook in React?

The useReducer hook in React is used to manage complex states in a component. It is an alternative to using the useState hook for managing a state, particularly when the state has multiple sub-values or requires complex updates. The useReducer hook allows developers to define a reducer function that handles state updates based on an action and returns a new state.

• What is the difference between a controlled component and an uncontrolled component in React?

In React, a controlled component is a component whose value is controlled by the parent component through props. The parent component has full control over the value of the component and can modify it at any time. An uncontrolled component, on the other hand, manages its own state internally and does not rely on the parent component for its value. Uncontrolled components are useful for handling user input that does not need to be validated or processed by the parent component.

• What is the difference between React and React Native?

React is a JavaScript library for building user interfaces for web applications, while React Native is a framework for building native mobile applications. React is focused on building reusable UI components that can be used across an application, while React Native provides a set of components and APIs optimized for mobile devices. Both React and React Native use similar syntax and core concepts, such as components and props, but the implementation details and use cases are different.

• What is the significance of the useEffect hook in React?

The useEffect hook in React is used to perform side effects in a functional component. It allows developers to perform tasks such as fetching data, setting up event listeners, or updating the DOM after a component has been rendered. The useEffect hook takes a function as its argument and is called after every render of the component. Developers can control when the useEffect hook is called by passing a dependency array as a second argument.

• What is the purpose of the useMemo hook in React?

The useMemo hook in React is used to memoize the result of a function and avoid unnecessary calculations. It takes a function and an array of dependencies as its arguments and returns a memoized value. If the dependencies do not change, the memoized value is returned, otherwise, the function is called again to recalculate the value. This can improve performance by reducing the number of calculations that need to be performed.

• What is the significance of the useCallback hook in React?

The useCallback hook in React is used to memoize a function and avoid unnecessary re-renders. It takes a function and an array of dependencies as its arguments and returns a memoized function. If the dependencies do not change, the memoized function is returned, otherwise, a new function is created. This can improve performance by reducing the number of re-renders that need to be performed.

• What is the purpose of the useRef hook in React?

The useRef hook in React is used to create a mutable reference that persists between renders. It allows developers to access a DOM node or a value that does not trigger a re-render when it changes. The useRef hook can be used to store values, access input elements, or create timers and intervals.

• What is the difference between server-side rendering and client-side rendering in React?

Server-side rendering is the process of rendering a web page on the server and sending a fully rendered HTML page to the client. This can improve performance and SEO by reducing the amount of JavaScript that needs to be downloaded and executed by the client. Client-side rendering, on the other hand, is the process of rendering a web page on the client side using JavaScript. This allows for dynamic content and interactivity, but can also result in slower load times and SEO challenges. React supports both server-side and client-side rendering, and developers can choose the approach that best suits their application’s needs.

• What is Redux, and how does it work with React?

Redux is a state management library for JavaScript applications. It provides a predictable state container that can be used to manage complex application states across different components and modules. Redux works with React by providing a set of APIs for connecting React components to the Redux store and dispatching actions to update the state. Developers can use Redux to manage the state in a centralized location and avoid passing state through props between components.

• What is JSX, and why is it used in React?

JSX is a syntax extension for JavaScript that allows developers to write HTML-like code inside JavaScript. It is used in React to define the structure and appearance of components. JSX is not a separate language or template engine but is translated into JavaScript by the Babel compiler. JSX can make code more readable and easier to maintain by allowing developers to write HTML-like code alongside JavaScript logic.