ReactJs Interview Questions

Introduction

React is a popular JavaScript library for building user interfaces, developed by Facebook. It's widely used for creating dynamic and responsive web applications.

Key Concepts in React

1. What is React?

React is an open-source JavaScript library for building user interfaces, particularly for single-page applications.

2. What is JSX?

JSX is a syntax extension for JavaScript that looks similar to XML or HTML. It's used with React to describe what the UI should look like.

3. What is the virtual DOM?

The virtual DOM is a lightweight copy of the actual DOM that React uses to optimize updates and improve performance.

4. What are components in React?

Components are the building blocks of a React application. They can be either class-based or functional.

5. What are props in React?

Props (short for properties) are read-only attributes used to pass data from parent to child components.

6. What is state in React?

State is a built-in React object used to contain data or information about the component that can change over time.

7. What is the difference between state and props?

State is local to the component and can be changed, while props are passed from parent to child components and are immutable.

8. What is a higher-order component (HOC)?

An HOC is a function that takes a component and returns a new component with additional props or functionality.

Advanced Concepts

9. What is the significance of keys in React?

Keys help React identify which items have changed, are added, or are removed and improve the performance of lists.

10. What are refs in React?

Refs provide a way to access the DOM nodes or React elements created in the render method.

11. What is the purpose of the setState function?

setState is used to update the component's state and re-render the component.

12. What is the Context API?

The Context API is a way to pass data through the component tree without having to pass props down manually at every level.

13. What are React hooks?

Hooks are functions that let you use state and other React features in functional components.

14. What is useEffect used for?

useEffect is a hook that allows you to perform side effects in function components.

15. What is the useReducer hook?

useReducer is a hook used for managing complex state logic in React applications.

16. What is the useMemo hook?

useMemo is a hook that memoizes the result of a function to improve performance by avoiding unnecessary calculations.

17. What is the useCallback hook?

useCallback is a hook that returns a memoized callback function, preventing it from being recreated on every render.

18. What is the useContext hook?

useContext is a hook that allows you to access the context value within a functional component.

19. What is React Router?

React Router is a library used for routing in React applications, enabling navigation among different views.

20. What is the use of the useRef hook?

useRef is a hook that returns a mutable ref object whose .current property is initialized to the passed argument.

21. What is an error boundary in React?

Error boundaries are components that catch JavaScript errors in their child component tree and display a fallback UI.

22. What is the Suspense component?

Suspense is a component that allows you to handle the loading of components asynchronously.

23. What is a React fragment?

React Fragments allow you to group multiple elements without adding extra nodes to the DOM.

24. What is the purpose of the forwardRef function?

forwardRef is a function that allows you to forward refs to child components.

25. What is a controlled component?

A controlled component is a component that renders form elements and controls them by keeping the form data in the component's state.

26. What is an uncontrolled component?

An uncontrolled component is a component that does not control its own state and relies on the DOM to maintain its state.

27. What is prop drilling?

Prop drilling is the process of passing data from parent to child components through props at multiple levels.

28. What is the significance of React.memo?

React.memo is a higher-order component that prevents unnecessary re-renders by memoizing the component.

29. What are React portals?

React portals provide a way to render children into a DOM node that exists outside the parent component's DOM hierarchy.

30. What is code-splitting in React?

Code-splitting is a technique used to split the application code into smaller chunks, which can be loaded on demand to improve performance.

31. What is lazy loading in React?

Lazy loading is a technique used to load components only when they are needed, reducing the initial load time of the application.

32. What is Redux?

Redux is a state management library for JavaScript applications, commonly used with React to manage the state in a predictable way.

33. What is a Redux store?

A Redux store is an object that holds the state of the application and provides methods to access and update the state.

34. What are actions in Redux?

Actions are plain JavaScript objects that describe an intention to change the state.

35. What are reducers in Redux?

Reducers are pure functions that take the current state and an action as arguments and return a new state.

36. What is the use of the connect function in Redux?

The connect function is used to connect a React component to the Redux store, allowing the component to access the state and dispatch actions.

Frequently Asked Questions (FAQs)

Q1. What is React?