ReactJS Interview Questions and Answers

Introduction

Preparing for a ReactJS interview can be daunting, whether you’re a fresher or experienced professional. This guide covers latest interview questions and answers to help you succeed.

Basics of React

Here’s a list of the frequently asked reactjs questions with answers on the technical concept which you can expect to face during an interview.

1) What is React?

React is a JavaScript library for building user interfaces, particularly single-page applications where data is dynamically updated.

2) What are the main features of React?

Some main features include JSX, Virtual DOM, components, one-way data binding, and high performance.

3) What is JSX?

JSX stands for JavaScript XML. It allows you to write HTML in React, making it easier to write and add HTML in React.

4) What is the Virtual DOM?

The Virtual DOM is a concept where a virtual representation of the real DOM is kept in memory and synced with the real DOM by a library such as ReactDOM.

5) What are components in React?

Components are independent and reusable bits of code. They serve the same purpose as JavaScript functions but work in isolation and return HTML via a render function.

State and Props

6)What is the difference between state and props?

State is a built-in React object used to contain data or information about the component, while props are inputs to a component that are passed down from a parent component.

Feature	`state`	`props`
Definition	A built-in React object used to contain data or information about the component.	Inputs to a component that are passed down from a parent component.
Mutability	State is mutable and can be changed by the component itself.	Props are immutable and cannot be changed by the component that receives them.
Management	Managed within the component using `this.setState()` or `useState` hook.	Managed by the parent component and passed down to child components.
Usage	Used to handle internal component data and control the component’s behavior.	Used to pass data and event handlers from a parent to a child component.
Example	A component that tracks user input or manages its own data.	A parent component passing a user name to a child component for display.

7)How do you update the state in React?

The state can be updated using the setState method or the useState hook in functional components.

8) What are the different ways to pass data between components?

Data can be passed between components using props, context API, or state management libraries like Redux.

9)What is the purpose of the key prop?

The key prop is a special attribute used in lists to help React identify which items have changed, are added, or are removed.

10)How can you conditionally render components in React?

Conditional rendering can be done using JavaScript operators like if, else, and ternary operators within JSX.

Lifecycle Methods and Hooks

11) What are React lifecycle methods?

Lifecycle methods are hooks that allow you to run code at specific points in a component’s lifecycle

12) What is the useEffect hook?

The useEffect hook lets you perform side effects in functional components. It is equivalent to lifecycle methods
For example

				
					useEffect( ()=>{
})

13) How do you use the useState hook?

The useState hook is used to add state to functional components. It returns an array with the current state and a function to update it.

				
					Const [variable,setfunction]=useState()

14) What is the useContext hook?

The useContext hook allows you to access the context within a functional component, eliminating the need for a consumer component.

15) What is the purpose of the useReducer hook?

The useReducer hook is used for managing complex state logic in functional components, acting as an alternative to useState.

Handling Events

16) How do you handle events in React?

Events in React are handled using camelCase syntax, and you pass an event handler function as the value.

17) What is a synthetic event in React?

Synthetic events are objects that wrap native events to ensure cross-browser compatibility.

18) How do you pass arguments to event handlers in React?

Arguments can be passed to event handlers by wrapping the handler in an arrow function or by using the bind method.

19) What is the useCallback hook?

The useCallback hook returns a memoized version of the callback function that only changes if one of the dependencies has changed.

20)What is the useMemo hook?

The useMemo hook is used to memoize expensive calculations, preventing unnecessary recalculations on re-renders.

Advanced Concepts

21)What is React Router?

React Router is a standard library for routing in React, enabling navigation among different components in a React application.
You have to install it by writing the following commands in the terminal
npm install react-router-dom

22)What is Redux?

Redux is a state management library often used with React to manage and centralize application state.

23) What is Context API?

The Context API is a React structure that allows you to share data across all levels of the application without passing props manually at every level.

24) How do you optimize performance in a React application?

Performance can be optimized by using techniques like code splitting, lazy loading, memoization, and avoiding unnecessary re-renders.

Miscellaneous

25) What are Higher-Order Components (HOCs)?

Higher-Order Components are functions that take a component and return a new component with additional props or behavior.

26) What are render props?

Render props are a technique for sharing code between React components using a prop whose value is a function.

27)What is the purpose of the Error Boundary?

Error Boundaries are React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of crashing the whole component tree.

28) How do you handle forms in React?

Forms in React are handled using controlled components where form data is managed by the component’s state.

29) What is the difference between controlled and uncontrolled components?

Controlled components have their form data controlled by the React component state, while uncontrolled components manage their own state internally.

Feature	Controlled Components	Uncontrolled Components
State Management	Form data is controlled by the React component state.	Form data is managed internally by the DOM.
Access to Data	Data is accessed and manipulated through React state.	Data is accessed using refs or direct DOM manipulation.
Form Updates	Updates are triggered by changes in React state.	Updates are handled internally by the DOM, independent of React state.
Control	Provides more control over form behavior and validation.	Less control over form behavior; suitable for simpler scenarios.
Example	A form where the input values are bound to React state and updated via event handlers.	A form where the input values are accessed directly via refs or by querying the DOM.

30) How do you create a React project using Create React App?

You can create a React project using Create React App by running npx create-react-app project-name in the terminal.

Conclusion

Mastering the questions and concepts outlined in this guide is essential for excelling in ReactJs interviews. By thoroughly preparing with these common interview questions and their answers, you’ll be well-equipped to showcase your expertise and confidence during the interview process.

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