30 React js Interview Questions and Answers for Beginners

React JS is one of the most popular JavaScript libraries used for developing user interfaces. It is maintained by Facebook and a community of developers. React JS is known for its simplicity and efficiency in building reusable UI components.

If you are preparing for a React JS interview, it is important to be familiar with some common interview questions. In this tutorial, we will go over the top 30 React JS interview questions and provide detailed answers for beginners.

1. What is React JS?

React JS is a JavaScript library for building user interfaces. It allows developers to create reusable UI components that can be easily updated when the underlying data changes.

2. What are the key features of React JS?

Some key features of React JS include:

– Virtual DOM: React uses a virtual representation of the DOM to improve performance.
– Reusable components: React allows developers to create reusable UI components.
– JSX: JSX is a syntax extension for JavaScript that allows developers to write HTML inside their JavaScript code.
– One-way data flow: React follows a one-way data flow, making it easier to manage data in the application.

3. What is JSX?

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

4. What is a component in React JS?

A component in React JS is a reusable UI element that can be easily reused throughout an application. Components can have their own state and properties.

5. What are key differences between functional components and class components in React?

Functional components are stateless and are defined as simple JavaScript functions, while class components have their own state and are defined using ES6 classes.

6. What is a state in React JS?

A state in React JS is an object that contains data that influences a component’s rendering. State allows React components to update and render dynamically based on changes in data.

7. What is props in React JS?

Props, short for properties, are used to pass data from parent components to child components in React. Props are read-only and are used to make components more reusable and maintainable.

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

State is used to manage internal component data that can change over time, while props are used to pass data from parent components to child components.

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

The componentDidMount() lifecycle method is called after a component has been mounted to the DOM. It is commonly used to fetch data from an API or set up event listeners.

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

Controlled components are components whose value is controlled by React state, while uncontrolled components manage their own state internally.

11. What is the purpose of the shouldComponentUpdate() lifecycle method in React?

The shouldComponentUpdate() lifecycle method is used to indicate whether a component should re-render based on changes in state or props. It can be used to optimize the rendering performance of a React component.

12. What is the purpose of the setState() method in React?

The setState() method is used to update the state of a React component. When the state is updated, React will re-render the component to reflect the changes.

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

The render() method is used to return the JSX representation of a React component. It defines the UI elements that will be rendered to the DOM.

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

Stateful components have their own state data that can change over time, while stateless components do not have state and are defined as simple functions.

15. What is a higher-order component in React?

A higher-order component (HOC) in React is a function that takes a component and returns a new component with enhanced functionality. HOCs are used to share code between components and add common behavior to multiple components.

16. What are props drilling in React?

Props drilling in React refers to the process of passing props down through multiple levels of nested components. It can make the code harder to maintain and can be avoided by using context or HOCs.

17. What is the useContext hook in React?

The useContext hook in React is used to access React context in functional components. It provides a way to share data between components without using props drilling.

18. What is the purpose of the useEffect hook in React?

The useEffect hook in React is used to perform side effects in functional components. It can be used to fetch data, set up subscriptions, or perform cleanup when a component is mounted, updated, or unmounted.

19. What is the purpose of the useRef hook in React?

The useRef hook in React is used to create a mutable reference to a DOM element or a value that persists across component re-renders. It can be used to access DOM elements, store previ oud values, or create a reference to a child component.

20. What is the useReducer hook in React?

The useReducer hook in React is used to manage complex state logic in functional components. It is an alternative to using the useState hook when state logic is more complex and involves multiple actions.

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

useMemo hook is used to memoize the result of a computationally expensive function, while the useCallback hook is used to memoize a callback function. useMemo is used for memoizing values, while useCallback is used for memoizing functions.

22. What is the difference between React Native and React JS?

React Native is a framework for building mobile applications using React, while React JS is a library for building web applications. React Native uses native components to render UI elements on mobile devices, while React JS uses HTML and CSS to render UI elements in web applications.

23. What is Redux in React?

Redux is a state management library for React that helps manage complex application state in a predictable way. It provides a centralized store for application state and allows components to subscribe to changes in the state.

24. What is the Redux Thunk middleware?

The Redux Thunk middleware is used to handle asynchronous actions in Redux. It allows action creators to return functions instead of objects, which can be used to fetch data asynchronously and dispatch multiple actions.

25. What is the React router library?

The React router library is used for handling routing in React applications. It allows developers to configure different routes for different components and manage the navigation flow of the application.

26. What are React hooks?

React hooks are functions that allow developers to use state and other React features in functional components. They provide a way to add stateful logic to functional components without using class components.

27. What are some advantages of using React JS?

Some advantages of using React JS include:

– Component reusability: React allows developers to create reusable UI components.
– Virtual DOM: React uses a virtual representation of the DOM to improve performance.
– One-way data flow: React follows a one-way data flow, making it easier to manage data in the application.
– Rich ecosystem: React has a large and active community of developers, and a vast ecosystem of libraries and tools.

28. What are some limitations of using React JS?

Some limitations of using React JS include:

– Steep learning curve: React has a steep learning curve, especially for beginners.
– JSX: JSX can be confusing for developers who are not familiar with the syntax.
– Lack of official documentation: Some developers find it challenging to find official documentation for React JS.

29. How do you pass data between components in React?

Data can be passed between components in React using props. Props are used to pass data from parent components to child components. State can also be used to manage data within a component and update it dynamically.

30. How can you optimize the performance of a React application?

Some strategies for optimizing the performance of a React application include:

– Using the React.memo() function to memoize components and prevent unnecessary re-renders.
– Using the useMemo and useCallback hooks to memoize values and functions.
– Splitting components into smaller, more manageable pieces to improve rendering performance.
– Using lazy loading to load components only when they are needed.
– Using third-party libraries like React Virtualized or React Window to optimize rendering performance.

In conclusion, React JS is a powerful library for building user interfaces in JavaScript. By familiarizing yourself with these top 30 React JS interview questions and their answers, you will be better prepared for a React JS interview. Remember to practice coding and building projects using React to solidify your understanding of the library. Good luck!