100 React JS Interview Questions and Answers – Ultimate Guide

React JS has become one of the most popular front-end JavaScript libraries in recent years. With its ability to create dynamic and interactive user interfaces, it has become a staple in web development. As a result, many companies are looking for React developers to join their teams. If you are preparing for a React JS interview, it is essential to be well-prepared with the top 100 React JS interview questions and answers. In this tutorial, we will cover a comprehensive list of questions that you might encounter in a React JS interview.

1. What is React JS?

React JS is an open-source JavaScript library developed by Facebook for building user interfaces, specifically for single-page applications. It allows developers to create reusable UI components, making it easier to build complex and interactive websites and web applications.

2. What are the key features of React JS?

Some of the 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 allows developers to write HTML-like syntax in JavaScript code, making it easier to build and maintain UI components.
– Components: React components are reusable and can be composed together to create complex UIs.
– Unidirectional data flow: React follows a unidirectional data flow, making it easier to manage data in the application.
– React Native: React Native is a framework that allows developers to build mobile applications using React JS.

3. What is JSX?

JSX stands for JavaScript XML, and it is a syntax extension for JavaScript that allows developers to write HTML-like code in their JavaScript files. JSX makes it easier to create and manipulate UI components in React.

4. What is the difference between virtual DOM and real DOM?

The virtual DOM is an in-memory representation of the actual DOM. When changes are made to the UI in React, React compares the virtual DOM with the real DOM and only updates the parts of the real DOM that have changed. This helps to improve performance and make React applications faster.

5. What are React components?

React components are independent and reusable pieces of code that represent a part of the user interface. Components can be composed together to create complex UIs, making it easier to build and maintain large-scale applications.

6. What is a functional component?

A functional component is a simple JavaScript function that returns JSX element. Functional components are stateless and do not have access to lifecycle methods.

7. What is a class component?

A class component is a JavaScript class that extends React.Component. Class components have access to lifecycle methods and can hold state, making them suitable for managing complex UI logic.

8. What is state in React?

State is a built-in feature in React that allows components to manage their data. State is mutable and can be changed over time, causing the UI to re-render.

9. What is props in React?

Props (short for properties) are a way to pass data from parent components to child components in React. Props are immutable and help to create dynamic and interactive UIs by passing data down the component tree.

10. What is the difference between state and props?

State is used to store data within a component, whereas props are used to pass data from parent to child components. State is mutable and can be changed, whereas props are immutable and cannot be changed by the child component.

11. What are lifecycle methods in React?

Lifecycle methods are special methods that get invoked at various stages of a component’s life cycle. Some of the most commonly used lifecycle methods in React include componentDidMount, componentDidUpdate, and componentWillUnmount.

12. What is the difference between functional and class components in terms of lifecycle methods?

Functional components do not have access to lifecycle methods, whereas class components have access to lifecycle methods such as componentDidMount, componentDidUpdate, and componentWillUnmount.

13. How do you handle events in React?

Events in React are handled using camelCase naming conventions in JSX. For example, onClick, onChange, onSubmit, etc. Event handlers are assigned to JSX elements using curly braces.

14. How do you handle forms in React?

Forms in React are handled using controlled components. Controlled components maintain their own state and update the state in response to user input, making it easier to manage form data and validate input.

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

Controlled components in React manage their own state and update the state in response to user input. Uncontrolled components do not manage their state and rely on the DOM to manage their state.

16. What is the difference between setState and forceUpdate in React?

setState is a built-in method in React used to update the component’s state and trigger a re-render. forceUpdate is a method that forces a re-render of the component without updating the state.

17. How do you update state in React?

State in React can be updated using the setState method. The setState method takes an object as an argument, which represents the updated state of the component.

18. What are keys in React?

Keys are special attributes in React that are used to help React identify which items have changed, been added, or been removed from a list. Keys are essential for performance optimization and should be unique within a list.

19. What is React Router?

React Router is a popular library for routing in React applications. It allows developers to create multiple pages in a single-page application and manage navigation between these pages.

20. What are higher-order components in React?

Higher-order components (HOCs) are a pattern in React that allows developers to reuse component logic by wrapping a component with another component. HOCs can be used to share code between components and add additional functionality to components.

21. What is the Context API in React?

The Context API is a feature in React that allows components to share data without having to pass props down the component tree manually. Context provides a way to pass data through the component tree without having to explicitly pass props down the component hierarchy.

22. What are hooks in React?

Hooks are a feature in React that allows developers to use state and other React features in functional components without having to write a class component. Hooks provide a way to reuse logic and stateful logic in functional components.

23. What are the basic hooks available in React?

Some of the basic hooks available in React include:
– useState: used to manage state in functional components.
– useEffect: used to perform side effects in functional components.
– useContext: used to access context in functional components.
– useRef: used to create a mutable reference in functional components.

24. What is lazy loading in React?

Lazy loading is a technique in React that allows components to be loaded asynchronously only when they are needed. Lazy loading helps to improve performance by loading only the required components and reducing the initial load time of the application.

25. How do you handle authentication in React applications?

Authentication in React applications is typically handled by using libraries such as Auth0, Firebase, or implementing custom authentication logic using local storage, cookies, or JSON Web Tokens (JWT). Authentication logic can be implemented using hooks, context API, or higher-order components.

26. What are the best practices for optimizing performance in React applications?

Some best practices for optimizing performance in React applications include:
– Using functional components and hooks instead of class components.
– Avoiding unnecessary re-renders by optimizing component lifecycle methods.
– Using lazy loading and code splitting to load components asynchronously.
– Implementing memoization for expensive operations.
– Using keys to optimize rendering lists.

27. How do you test React components?

React components can be tested using tools like Jest and Enzyme. Unit tests can be written to test individual components, while integration tests can be written to test the interactions between different components.

28. What is server-side rendering in React?

Server-side rendering (SSR) is a technique that allows React applications to render on the server before sending it to the client. SSR helps to improve performance and SEO by delivering pre-rendered HTML to the client.

29. What are portals in React?

Portals in React are a feature that allows developers to render a component outside the parent DOM hierarchy. Portals are useful for rendering components like modals or tooltips that need to be rendered outside the normal DOM structure.

30. How do you handle state management in large React applications?

State management in large React applications can be handled using libraries like Redux, MobX, or the Context API. These libraries provide a way to manage global state across multiple components and make it easier to handle complex state logic.

31. What is Redux?

Redux is a popular state management library for JavaScript applications, including React. Redux helps to manage the global state of an application by storing state in a single store and dispatching actions to update the state.

32. How does Redux work in a React application?

In a React application, Redux is typically integrated using the react-redux library, which provides bindings to connect Redux with React components. State is stored in a single store, actions are dispatched to update the state, and components can subscribe to the store to access the state.

33. What are the basic principles of Redux?

Some of the basic principles of Redux include:
– Single source of truth: All application state is stored in a single store.
– State is read-only: State can only be updated by dispatching actions.
– Changes are made with pure functions: Reducers are pure functions that take the current state and an action and return the new state.

34. How do you connect Redux with React?

Redux can be connected with React using the react-redux library, which provides the connect and Provider components. The connect component is used to connect Redux state to React components, while the Provider component is used to provide the Redux store to the entire application.

35. What are actions and reducers in Redux?

Actions are payloads of information that send data from the application to the Redux store. Reducers are functions that specify how the state changes in response to an action. Reducers take the current state and an action and return the new state.

36. What is the difference between Redux and Context API?

Redux and the Context API are both used for state management in React applications. Redux is a standalone library that provides a global state management solution, while the Context API is a feature built into React that provides a way to share data between components without using props.

37. How do you test Redux applications?

Redux applications can be tested using tools like Jest and Enzyme, similar to testing React components. Unit tests can be written to test actions and reducers, while integration tests can be written to test the interactions between components and Redux.

38. What is the difference between React and Angular?

React and Angular are both popular front-end frameworks for building web applications. React is a JavaScript library developed by Facebook, while Angular is a full-fledged front-end framework developed by Google. React is more lightweight and focused on building UI components, while Angular provides a full set of tools for building large-scale applications.

39. What is the difference between React and Vue.js?

React and Vue.js are both popular front-end libraries for building user interfaces. React is developed by Facebook and is more focused on building UI components, while Vue.js is a progressive JavaScript framework that offers a more opinionated approach to building applications. Vue.js is easier to learn and offers a more flexible and scalable architecture compared to React.

40. What is the difference between class components and functional components?

Class components are JavaScript classes that extend React.Component and have access to lifecycle methods and state. Functional components are simple JavaScript functions that return JSX elements and are stateless. Functional components are easier to read and debug compared to class components.

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

Controlled components in React manage their own state and update the state in response to user input. Uncontrolled components do not manage their state and rely on the DOM to manage their state. Controlled components are preferred in React applications for better control over form data.

42. What is the significance of key in React?

Keys are special attributes in React that help React identify which items have changed, been added, or been removed from a list. Keys are essential for performance optimization and should be unique within a list to ensure efficient rendering.

43. What is the purpose of the render method in React?

The render method in React is responsible for rendering the UI of a component. The render method returns a React element, which is a lightweight representation of the DOM node. The render method is called whenever the component needs to be re-rendered.

44. What is the purpose of propTypes in React?

PropTypes are used in React to specify the data type of props that a component expects to receive. PropTypes help to validate the data passed to a component and can help to catch bugs early in development.

45. What are the advantages of using React?

Some of the advantages of using React 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 allows developers to write HTML-like syntax in JavaScript code, making it easier to build and maintain UI components.
– Components: React components are reusable and can be composed together to create complex UIs.

46. What are the disadvantages of using React?

Some of the disadvantages of using React include:
– Steeper learning curve: React has a steeper learning curve compared to other front-end libraries like Vue.js.
– Boilerplate code: React requires more boilerplate code to set up compared to other libraries.
– Lack of built-in features: React is a library and not a full-fledged framework, which means developers need to rely on third-party libraries for certain features.

47. What is the React Developer Tools extension?

The React Developer Tools extension is a browser extension that allows developers to inspect the React component hierarchy, monitor component state and props, and debug React applications. The extension is available for Chrome and Firefox and is a useful tool for debugging React applications.

48. What is the purpose of using fragments in React?

Fragments are a feature in React that allows developers to return multiple elements from a component without the need for a wrapping element. Fragments help to keep the DOM structure clean and maintain performance by reducing unnecessary nesting.

49. How do you handle errors in React?

Errors in React can be handled using JavaScript try/catch blocks or by using error boundaries. Error boundaries are special components in React that catch JavaScript errors anywhere in their child component tree and log those errors or display a fallback UI.

50. What is the purpose of the shouldComponentUpdate method in React?

The shouldComponentUpdate method is a lifecycle method in React that allows developers to optimize performance by preventing unnecessary re-renders. The shouldComponentUpdate method is called before re-rendering a component and allows developers to control when a component should update.

51. What is the purpose of the getDerivedStateFromProps method in React?

The getDerivedStateFromProps method is a lifecycle method in React that allows developers to update the state of a component based on changes in props. The getDerivedStateFromProps method is called before rendering and can be used to update the state based on new props.

52. What is the purpose of the getSnapshotBeforeUpdate method in React?

The getSnapshotBeforeUpdate method is a lifecycle method in React that allows developers to capture information about the DOM before it is updated. The getSnapshotBeforeUpdate method is called before a component is updated and can be used to capture information like scroll position or DOM state.

53. What are error boundaries in React?

Error boundaries are special components in React that catch JavaScript errors anywhere in their child component tree and log those errors or display a fallback UI. Error boundaries help to prevent crashes and provide a better user experience by handling errors gracefully.

54. What is the purpose of useMemo in React?

useMemo is a hook in React that memoizes a value and returns a memoized value. useMemo can be used to optimize performance by memoizing expensive computations and only recomputing the value when the dependencies change.

55. What is the purpose of useCallback in React?

useCallback is a hook in React that memoizes a callback function and returns a memoized version of the function. useCallback can be used to optimize performance by preventing unnecessary renders of components that depend on the callback function.

56. What is the purpose of useContext in React?

useContext is a hook in React that allows components to access the context data provided by the nearest Context.Provider component. useContext can be used to avoid prop drilling and make it easier to share data between components.

57. What is the purpose of useReducer in React?

useReducer is a hook in React that allows developers to manage complex state logic in functional components. useReducer takes a reducer function and an initial state and returns the current state and a dispatch function to update the state.

58. What is the purpose of useLayoutEffect in React?

useLayoutEffect is a hook in React that fires synchronously after all DOM mutations have been processed. useLayoutEffect is similar to useEffect, but it fires synchronously and can be used to perform DOM measurements or updates after a component has rendered.

59. What is the purpose of useImperativeHandle in React?

useImperativeHandle is a hook in React that allows functional components to customize the instance value that is exposed to parent components when using the ref attribute. useImperativeHandle can be used to expose specific methods or properties to the parent component.

60. What is the purpose of useDebugValue in React?

useDebugValue is a hook in React that provides a custom label for a custom hook when debugging in the React DevTools. useDebugValue can help developers provide more useful information when debugging custom hooks.

61. What is the purpose of useEffect in React?

useEffect is a hook in React that allows developers to perform side effects in functional components. useEffect is similar to lifecycle methods in class components and can be used to perform data fetching, subscribing to events, or updating the DOM.

62. What is the purpose of useRef in React?

useRef is a hook in React that creates a mutable reference and returns a mutable ref object. useRef can be used to access DOM elements, store values between renders, or interact with third-party libraries that require a reference.

63. What is the purpose of useReducer in React?

useReducer is a hook in React that allows developers to manage complex state logic in functional components. useReducer takes a reducer function and an initial state and returns the current state and a dispatch function to update the state.

64. What is the purpose of forwardRef in React?

forwardRef is a higher-order component in React that allows developers to access the ref of a child component from a parent component. forwardRef can be used to forward refs from parent to child components and access the underlying DOM node.

65. What is the purpose of useMemo in React?

useMemo is a hook in React that memoizes a value and returns a memoized value. useMemo can be used to optimize performance by memoizing expensive computations and only recomputing the value when the dependencies change.

66. What is the purpose of useCallback in React?

useCallback is a hook in React that memoizes a callback function and returns a memoized version of the function. useCallback can be used to optimize performance by preventing unnecessary renders of components that depend on the callback function.

67. What is the purpose of useContext in React?

useContext is a hook in React that allows components to access the context data provided by the nearest Context.Provider component. useContext can be used to avoid prop drilling and make it easier to share data between components.

68. What is the purpose of useLayoutEffect in React?

useLayoutEffect is a hook in React that fires synchronously after all DOM mutations have been processed. useLayoutEffect is similar to useEffect, but it fires synchronously and can be used to perform DOM measurements or updates after a component has rendered.

69. What is the purpose of useImperativeHandle in React?

useImperativeHandle is a hook in React that allows functional components to customize the instance value that is exposed to parent components when using the ref attribute. useImperativeHandle can be used to expose specific methods or properties to the parent component.

70.