2024 – Top 100 React JS Interview Questions and Answers

React JS has become one of the most popular front-end frameworks for building dynamic and interactive user interfaces. As a result, there is a high demand for React JS developers in the job market. If you are preparing for a React JS interview, it is important to be familiar with the most commonly asked questions. In this tutorial, we will go through the top 100 React JS interview questions and provide detailed answers for each.

1. What is React JS?

React JS is an open-source front-end JavaScript library developed by Facebook for building user interfaces or UI components. It allows developers to create efficient and reusable components and provides a virtual DOM for better performance.

2. What are the key features of React JS?

Some key features of React JS include:
– Virtual DOM: React uses a virtual DOM to improve performance by updating only the parts of the actual DOM that have changed.
– JSX: React uses JSX (JavaScript XML) syntax to write HTML in JavaScript code.
– Component-based architecture: React allows developers to create reusable and modular components that can be easily composed to build complex UIs.
– One-way data binding: React follows a unidirectional data flow, where data flows from parent components to child components.
– React hooks: React hooks are functions that allow developers to use state and other React features in functional components.

3. What is JSX?

JSX (JavaScript XML) is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript. JSX makes it easier to write and maintain React components by combining HTML and JavaScript in a single file.

4. What is a component in React?

A component in React is a reusable and independent piece of UI that encapsulates the HTML, CSS, and JavaScript logic needed to render a specific part of the user interface. Components can be composed together to build complex UIs.

5. What are the different types of components in React?

There are two main types of components in React:
– Functional components: Functional components are defined as JavaScript functions and are typically used for presentational or stateless components.
– Class components: Class components are ES6 classes that extend the React.Component class and are used for components that require state or lifecycle methods.

6. What is the difference between state and props in React?

– State is a built-in object in React that allows components to store and manage their own data. State is mutable and can be changed within the component.
– Props (short for properties) are read-only data passed from parent components to child components. Props are used to pass data from one component to another.

7. How is data binding implemented in React?

In React, data binding is achieved through props and state. Props are used to pass data from parent components to child components, while state is used to manage local component data.

8. What are React hooks?

React hooks are functions that allow developers to use state and other React features in functional components. Some common React hooks include useState, useEffect, useContext, and useReducer.

9. What is the useEffect hook in React?

The useEffect hook in React is used to perform side effects in functional components. Side effects can include data fetching, subscriptions, or DOM manipulation. The useEffect hook runs after every render and is used to manage effects that might require cleanup.

10. What is the useState hook in React?

The useState hook in React is used to add state to functional components. useState returns an array with two elements: the current state value and a function to update the state value. useState is used to manage component-specific state within functional components.

11. What is JSX in React?

JSX (JavaScript XML) is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript. JSX makes it easier to write and maintain React components by combining HTML and JavaScript in a single file.

12. How does React enable programmers to efficiently update user interfaces?

React uses a virtual DOM to efficiently update user interfaces. When the state of a React component changes, React creates a new virtual DOM representation of the component and compares it with the previous virtual DOM representation. React then calculates the minimal number of DOM mutations needed to update the actual DOM, which results in better performance.

13. How does React handle data binding?

React handles data binding by using props and state. Props are used to pass data from parent components to child components, while state is used to manage local component data. By combining props and state, React enables developers to create dynamic and interactive user interfaces.

14. What is the difference between controlled and uncontrolled components in React?

– Controlled components: In controlled components, the form data is controlled by React state. When the user interacts with the form elements, the React state is updated, resulting in a re-render of the component.
– Uncontrolled components: In uncontrolled components, the form data is controlled by the DOM. The form data is accessed using refs, and React does not manage the form data. Uncontrolled components are typically used when integration with non-React libraries is required.

15. What is the purpose of the render() method in React?

The render() method in React is a required method that returns the JSX or HTML markup of a component. The render() method is called whenever the component needs to be updated or re-rendered due to a change in state or props.

16. What are higher-order components in React?

Higher-order components (HOCs) are functions that take a component as an argument and return a new component. HOCs are used to enhance the functionality of existing components by adding additional logic or behavior. HOCs are a common pattern in React for code reuse and abstraction.

17. What are keys in React?

Keys in React are special attributes that provide a unique identity to child elements in a list. Keys help React identify which items have changed, added, or removed in a list of elements. Keys are used to improve the efficiency of rendering and updating lists in React.

18. What are fragments in React?

Fragments are a feature in React that allows developers to group multiple elements together without adding extra nodes to the HTML output. Fragments are represented by an empty tag <>… or … and are used to avoid adding unnecessary div elements in the DOM.

19. How can you pass data from a parent component to a child component in React?

Data can be passed from a parent component to a child component in React using props. Props are passed as attributes in the JSX code and can be accessed by the child component using the props object.

20. How can you update the state of a component in React?

The state of a component can be updated in React using the setState() method. The setState() method accepts an object as an argument and updates the state of the component with the new values. When the state is updated, React triggers a re-render of the component.

21. What is a React ref?

A ref in React is a way to access and interact with DOM elements or React components directly. Refs are created using the useRef() hook or the ref attribute in class components. Refs are typically used to access form elements, manage focus, or interact with third-party libraries.

22. What is the purpose of the key prop in React?

The key prop in React is used to provide a unique identity to child elements in a list. Keys help React identify which items have changed, added, or removed in a list of elements. Keys are essential for efficient rendering and updating of lists in React.

23. How can you handle events in React?

Events in React are handled using event handlers, which are functions that are executed when a specific event occurs. Event handlers are typically passed as props to React components and are triggered when the corresponding event occurs, such as a button click or form submission.

24. What is the purpose of the constructor() method in React?

The constructor() method in React is a special method that is called when an instance of a class component is created. The constructor() method is used to initialize the component’s state and bind event handlers. In modern React, using the constructor() method is not required, as state can be initialized using class properties.

25. How can you handle forms in React?

Forms in React can be handled using controlled components, where the form data is controlled by React state. Form input elements such as text inputs, checkboxes, and radio buttons are controlled by state, and their values are updated by event handlers. When the form is submitted, the form data is sent to a function or API for processing.

26. What are the different lifecycle methods in React?

React class components have several lifecycle methods that are called at different stages of a component’s lifecycle. Some common lifecycle methods include componentDidMount(), componentDidUpdate(), componentWillUnmount(), and shouldComponentUpdate(). These methods allow developers to perform actions such as fetching data, updating the DOM, or cleaning up resources.

27. What is the purpose of the componentDidMount() method in React?

The componentDidMount() method in React is a lifecycle method that is called after a component has been mounted or rendered to the DOM. The componentDidMount() method is used to perform side effects, such as data fetching, DOM manipulation, or setting up event listeners. The componentDidMount() method is a good place to fetch data from an API or perform initialization tasks.

28. What is the purpose of the componentDidUpdate() method in React?

The componentDidUpdate() method in React is a lifecycle method that is called after the component has been updated or re-rendered. The componentDidUpdate() method is used to perform side effects when the component’s state or props have changed. The componentDidUpdate() method is a good place to update the DOM based on new data or trigger side effects based on state changes.

29. What is the purpose of the componentWillUnmount() method in React?

The componentWillUnmount() method in React is a lifecycle method that is called just before a component is unmounted or removed from the DOM. The componentWillUnmount() method is used to perform cleanup tasks, such as unsubscribing from event listeners, clearing timers, or releasing resources. The componentWillUnmount() method is a good place to clean up any resources used by the component.

30. 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 re-rendered. The shouldComponentUpdate() method is used to optimize performance by preventing unnecessary re-renders of components. By returning false from shouldComponentUpdate(), developers can prevent the component from re-rendering when its state or props have not changed.

31. What is Redux?

Redux is a state management library for JavaScript applications, particularly those built using React or Angular. Redux provides a predictable state container that allows developers to manage the state of an application in a single location. Redux is based on the principles of a unidirectional data flow and immutability.

32. What are the three principles of Redux?

The three principles of Redux are:
– Single source of truth: The state of an application is stored in a single object called the store.
– State is read-only: The state of an application can only be changed by dispatching actions.
– Changes are made with pure functions: Reducers are pure functions that take the current state and an action as input and return a new state.

33. What are actions in Redux?

Actions in Redux are plain JavaScript objects that represent an intent to change the state of an application. Actions have a type property that describes the type of action being performed and can also include additional data or payload. Actions are dispatched to the Redux store using the dispatch() method.

34. What are reducers in Redux?

Reducers in Redux are pure functions that take the current state and an action as input and return a new state. Reducers are responsible for handling the dispatched actions and updating the state of the application accordingly. Reducers are used to maintain the immutability of the Redux state.

35. What is a Redux store?

A Redux store is an object that holds the state of an application and provides methods for accessing and updating the state. The Redux store is created using the createStore() function from the Redux library and is typically passed to the root component of a React application using the component.

36. What is the purpose of the connect() function in Redux?

The connect() function in Redux is a higher-order component that is used to connect a React component to the Redux store. The connect() function takes two arguments: mapStateToProps and mapDispatchToProps, which are used to map the state and actions from the Redux store to the props of the component. The connect() function enhances the functionality of the component by providing access to the Redux state and actions.

37. What is a middleware in Redux?

Middleware in Redux is a function that sits between the action dispatch and the reducer and allows developers to apply additional logic or side effects to the Redux workflow. Middleware can be used for tasks such as logging, asynchronous actions, or routing. Middleware is implemented using the applyMiddleware() function from the Redux library.

38. What is the purpose of the bindActionCreators() function in Redux?

The bindActionCreators() function in Redux is a utility function that is used to bind action creators to the dispatch() method and create an object that contains the bound action creators. The bindActionCreators() function simplifies the process of dispatching actions in Redux by automatically binding the action creators to the dispatch method.

39. What is the purpose of the combineReducers() function in Redux?

The combineReducers() function in Redux is used to combine multiple reducers into a single reducer function that can be passed to the createStore() function. The combineReducers() function takes an object whose keys represent the state slices and values represent the reducer functions that handle those slices. The combineReducers() function simplifies the process of managing multiple slices of state in a Redux application.

40. How can you integrate React with Redux?

React can be integrated with Redux by using the react-redux library, which provides bindings to connect React components to the Redux store. React components can be connected to the Redux store using the connect() function, which maps the Redux state and actions to the props of the component.

41. What is the purpose of the dispatch() method in Redux?

The dispatch() method in Redux is used to dispatch actions to the Redux store, which triggers the reducers to update the state of the application. Actions are dispatched by calling the dispatch() method with an action object as an argument. The dispatch() method is typically used in event handlers or lifecycle methods of React components.

42. What is React Router?

React Router is a popular library for routing in React applications. React Router provides components and hooks for creating and managing routes in a single-page application. React Router enables developers to create a declarative routing configuration using JSX and manage routing logic within React components.

43. What are the three main components of React Router?

The three main components of React Router are:
– BrowserRouter: A component that provides the browser history API and enables declarative routing in a React application.
– Route: A component that renders UI based on the current URL path.
– Link: A component that provides declarative navigation by generating anchor tags with proper href attributes.

44. What is the purpose of the BrowserRouter component in React Router?

The BrowserRouter component in React Router is used to provide the browser history API and enable declarative routing in a React application. The BrowserRouter component creates a new history instance and passes it down to all other route components, enabling navigation using browser history.

45. What is the purpose of the Route component in React Router?

The Route component in React Router is used to render UI based on the current URL path. The Route component takes a path and a component as props and renders the component when the current URL matches the specified path. Nested Route components can be used to create nested routes within a React application.

46. What is the purpose of the Link component in React Router?

The Link component in React Router is used to provide declarative navigation by generating anchor tags with proper href attributes. The Link component enables developers to create links between different routes in a React application without triggering a full page reload. The Link component is a common way to handle client-side navigation in a single-page application.

47. How can you retrieve URL parameters in React Router?

URL parameters in React Router can be retrieved using the useParams() hook provided by React Router. The useParams() hook returns an object containing the key-value pairs of the URL parameters specified in the path of the Route component.

48. What are nested routes in React Router?

Nested routes in React Router are routes that are defined within the component hierarchy of a React application. Nested routes allow developers to create complex route configurations by nesting Route components within each other. Nested routes are useful for creating multi-level navigation structures in a React application.

49. How can you navigate programmatically in React Router?

Programmatic navigation in React Router can be achieved using the useHistory() hook provided by React Router. The useHistory() hook returns a history object that can be used to navigate to a different route by calling the push() method with the desired route path as an argument.

50. What is lazy loading in React Router?

Lazy loading in React Router is a technique used to dynamically load components only when they are needed. Lazy loading improves the performance of a React application by reducing the initial load time and splitting the code into smaller, more manageable chunks. React Router provides a lazy() function to enable lazy loading of components.

51. What is server-side rendering in React?

Server-side rendering (SSR) in React is a technique used to render React components on the server and send the rendered HTML to the client. SSR improves the performance and SEO of a React application by reducing the initial load time and enabling search engines to index the content of the application.

52. What are the benefits of server-side rendering in React?

Some benefits of server-side rendering in React include:
– Improved performance: SSR reduces the initial load time of a React application by pre-rendering the content on the server.
– SEO optimization: SSR enables search engines to crawl and index the content of a React application for better search engine rankings.
– Accessibility: SSR ensures that the content is accessible to users with JavaScript disabled or using assistive technologies.
– Progressive enhancement: SSR provides a solid foundation for building progressively enhanced web applications.

53. What are the challenges of server-side rendering in React?

Some challenges of server-side rendering in React include:
– Complex configuration: Setting up server-side rendering in React requires additional configuration and tooling, which can be challenging for beginners.
– Server-side dependencies: SSR requires server-side dependencies and infrastructure to render React components on the server, which adds complexity to the application.
– Performance trade-offs: SSR can introduce performance trade-offs, such as increased server load and slower initial load times for complex applications.

54. What is error boundaries in React?

Error boundaries in React are components that catch JavaScript errors in the component tree below them and display a fallback UI instead of crashing the entire application. Error boundaries are used to handle errors gracefully and prevent the application from crashing due to unhandled exceptions.

55. How can you create an error boundary in React?

You can create an error boundary in React by defining a class component that implements the componentDidCatch() lifecycle method. The componentDidCatch() method is called when an error occurs in the component tree below the error boundary. Inside the componentDidCatch() method, you can update the state of the error boundary component to display an error message or fallback UI.

56. What is the purpose of the useReducer hook in React?

The useReducer hook in React is used to manage complex state logic in functional components. useReducer takes a reducer function and an initial state as arguments and returns the current state and a dispatch function. useReducer is often used as an alternative to useState for managing more complex state updates.

57. What is the difference between useCallback and useMemo in React?

– useCallback: useCallback is used to memoize a function and prevent unnecessary re-renders of child components. useCallback returns a memoized version of a function that only changes if the dependencies specified in the dependency array change.
– useMemo: useMemo is used to memoize a value and prevent unnecessary re-computations of expensive calculations. useMemo returns a memoized value that is recalculated only when the dependencies specified in the dependency array change.