React JS Questions and Answers

The widely used JavaScript library for writing web user interfaces quickly and interactively has been React JS. Started by Facebook, now known as Meta, this service is widely used for reusing components in creating many web applications. It thus follows a component-based approach to architecture, which naturally ensures efficiency and modularity in development.

React also offers key features such as JSX that enables developers to write the components as elements, virtual DOM that updates the UI efficiently, and hooks like state management using useState and handling lifecycle events using useEffect on functional components. Developers build applications with these concepts involving props for passing data to other components, React Router for navigation, and state management using the Context API.

React is one of the must skills for web developers, with great ease of use and robust ecosystem. This article consists of some important multiple-choice questions that will help you master core concepts in the following areas: React components, event handling, state management, and optimization techniques.

Discover how React enables developers to build dynamic and scalable applications, optimize performance using tools like React.memo and useCallback, and leverage state management tools such as Redux. Dive into these questions to strengthen your React knowledge and stand out in the competitive web development landscape.

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#1. What is React JS?

A CSS framework

A web server

A database query language

A JavaScript library for building user interfaces

#2. React was developed by which company?

Amazon

Microsoft

Google

Facebook (now Meta)

#3. Which of the following is used to create a React component?

Objects only

Arrays

JSON

Class or Function

#4. How does React JS update the UI efficiently?

By directly manipulating the real DOM

By creating new DOM nodes

By using HTTP requests

By using a virtual DOM

#5. What is JSX?

A React plugin

A special syntax for CSS

A tool to compile React code

JavaScript XML

#6. Which method is used to create components in React?

React.createElement()

React.newComponent()

React.addComponent()

React.build()

#7. How can you handle data in React?

By defining global variables

By using MySQL

By calling REST APIs directly in JSX

By using props and state

#8. What is the purpose of React’s useState hook?

To manage lifecycle events

To manage state in functional components

To handle API requests

To create reusable components

#9. React follows which programming concept?

Functional Programming

Procedural Programming

Component-Based Architecture

Object-Oriented Programming

#10. What is the default value of this.state in a React class component?

An empty object { }

undefined

[ ]

null

#11. What does the render() method do in React?

It returns JSX to render the UI

It fetches data from APIs

It binds event handlers

It updates the browser URL

#12. How do you pass data between React components?

By using AJAX

By using cookies

By defining variables globally

Using props

#13. What is React Router used for?

For enabling navigation in a React app

For styling components

For accessing APIs

For managing databases

#14. What does key prop do in React lists?

Adds a class to each list item

Sorts the list automatically

Enables drag-and-drop functionality

Helps React identify which items have changed

#15. Which of the following is NOT a lifecycle method?

componentWillUnmount()

componentDidMount()

componentDelete()

shouldComponentUpdate()

#16. What is the use of the useEffect hook?

To fetch data

To manage state

To create components

To handle side effects in functional components

#17. What is React’s context API used for?

For making REST API calls

To replace Redux entirely

To pass data deeply without prop drilling

For styling components

#18. Which command creates a new React application?

npx create-react-app

create-react

react-new-app

npm install react-app

#19. How do you handle events in React?

By using window.event

By using inline JavaScript

By passing a function as a prop

By defining event listeners globally

#20. What is the default behavior of React when a state changes?

It triggers an API call

It fetches new data

It logs a warning

It re-renders the component

#21. What does React.memo do?

Converts a functional component into a class component

Handles side effects

Optimizes rendering by memoizing the output

Makes components asynchronous

#22. How do you conditionally render components in React?

By using switch statements

By using React.createElement()

By modifying the DOM directly

By using ternary operators or if statements

#23. How can you optimize a React app?

Using React.memo and useCallback hooks

Using multiple render() calls

Using global variables

Avoiding functional components

#24. What is React’s StrictMode used for?

To improve performance

To highlight potential issues in an application

To enable animations

To manage app state

#25. Which statement is TRUE about React?

React only supports HTML pages.

React can be used to build mobile apps via React Native.

React is a backend framework.

React does not allow the use of JavaScript.

#26. What is the output of React.createElement(“div”, null, “Hello World”)?

An empty DOM node

A syntax error

A direct HTML element

A virtual DOM node representing a div element

#27. What is PropTypes in React?

A way to optimize rendering

A way to validate props passed to a component

A hook for side effects

A state management tool

#28. What does the useReducer hook do?

Manages complex state logic

Replaces CSS files

Generates virtual DOM nodes

Creates components

#29. Which hook would you use to fetch data from an API?

useEffect

useState

useReducer

useMemo

#30. What happens if setState is called inside render()?

It throws an error

It causes an infinite loop

It updates the state instantly

It does nothing

#31. What is the role of ReactDOM.render() in a React app?

To render a React element into the DOM

To fetch API data

To manage lifecycle methods

To update the React state

#32. What is the primary purpose of Redux in React?

To handle DOM manipulation

To render JSX efficiently

To manage global state

To replace React Router

#33. What does the useRef hook provide in React?

A method to fetch data

A way to access DOM nodes directly

State management

A debugging tool

#34. Which statement is TRUE about React’s useMemo?

It is used to create virtual DOM nodes.

It handles API calls automatically.

It replaces React.memo.

It memoizes the result of a computation to avoid unnecessary re-renders.

#35. What is React’s useCallback used for?

To update the virtual DOM

To memoize callback functions

To bind event handlers

To manage API requests

#36. What does the children prop in React represent?

The lifecycle methods of a component

All the state variables of a component

The CSS styles applied to a component

The nested elements inside a React component

#37. What is the correct way to prevent a default event in React?

return false

event.preventDefault()

event.stopPropagation()

event.prevent()

#38. What does the setState() function do in React class components?

Modifies the props

Directly modifies the state variable

Deletes the state variable

Updates the state and triggers a re-render

#39. Which React hook replaces lifecycle methods like componentDidMount and componentDidUpdate?