React JS

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 web server

A CSS framework

A database query language

A JavaScript library for building user interfaces

#2. React was developed by which company?

Amazon

Google

Facebook (now Meta)

Microsoft

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

JSON

Arrays

Class or Function

Objects only

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

By using HTTP requests

By directly manipulating the real DOM

By using a virtual DOM

By creating new DOM nodes

#5. What is JSX?

A special syntax for CSS

A React plugin

JavaScript XML

A tool to compile React code

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

React.createElement()

React.build()

React.addComponent()

React.newComponent()

#7. How can you handle data in React?

By defining global variables

By using props and state

By calling REST APIs directly in JSX

By using MySQL

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

To manage state in functional components

To create reusable components

To handle API requests

To manage lifecycle events

#9. React follows which programming concept?

Functional Programming

Object-Oriented Programming

Component-Based Architecture

Procedural Programming

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

null

[ ]

An empty object { }

undefined

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

It updates the browser URL

It fetches data from APIs

It binds event handlers

It returns JSX to render the UI

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

By defining variables globally

By using cookies

Using props

By using AJAX

#13. What is React Router used for?

For styling components

For managing databases

For accessing APIs

For enabling navigation in a React app

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

Helps React identify which items have changed

Adds a class to each list item

Sorts the list automatically

Enables drag-and-drop functionality

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

componentWillUnmount()

componentDidMount()

componentDelete()

shouldComponentUpdate()

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

To handle side effects in functional components

To fetch data

To manage state

To create components

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

For making REST API calls

To pass data deeply without prop drilling

To replace Redux entirely

For styling components

#18. Which command creates a new React application?

react-new-app

npx create-react-app

npm install react-app

create-react

#19. How do you handle events in React?

By using window.event

By defining event listeners globally

By passing a function as a prop

By using inline JavaScript

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

It fetches new data

It triggers an API call

It re-renders the component

It logs a warning

#21. What does React.memo do?

Makes components asynchronous

Optimizes rendering by memoizing the output

Handles side effects

Converts a functional component into a class component

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

By modifying the DOM directly

By using switch statements

By using React.createElement()

By using ternary operators or if statements

#23. How can you optimize a React app?

Using global variables

Avoiding functional components

Using React.memo and useCallback hooks

Using multiple render() calls

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

To enable animations

To highlight potential issues in an application

To improve performance

To manage app state

#25. Which statement is TRUE about React?

React is a backend framework.

React only supports HTML pages.

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

React does not allow the use of JavaScript.

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

A syntax error

A direct HTML element

An empty DOM node

A virtual DOM node representing a div element

#27. What is PropTypes in React?

A hook for side effects

A way to optimize rendering

A way to validate props passed to a component

A state management tool

#28. What does the useReducer hook do?

Generates virtual DOM nodes

Replaces CSS files

Manages complex state logic

Creates components

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

useState

useReducer

useMemo

useEffect

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

It throws an error

It causes an infinite loop

It does nothing

It updates the state instantly

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

To update the React state

To render a React element into the DOM

To fetch API data

To manage lifecycle methods

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

To render JSX efficiently

To handle DOM manipulation

To manage global state

To replace React Router

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

A debugging tool

A method to fetch data

A way to access DOM nodes directly

State management

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

It is used to create virtual DOM nodes.

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

It replaces React.memo.

It handles API calls automatically.

#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 nested elements inside a React component

All the state variables of a component

The lifecycle methods of a component

The CSS styles applied to a component

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

event.preventDefault()

event.stopPropagation()

event.prevent()

return false

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

Directly modifies the state variable

Deletes the state variable

Updates the state and triggers a re-render

Modifies the props

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

useRef

useReducer

useState

useEffect

#40. What is React.Fragment used for?

To handle API requests

To wrap multiple components with styling

To group multiple children without adding extra DOM nodes

To modify the virtual DOM

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