Here are 20 ReactJS interview questions along with their answers: 1. What is ReactJS? ReactJS is a popular JavaScript library used for building user interfaces. It allows developers to create reusable UI components and efficiently update the UI in response to changes in data. 2. What are the advantages of using ReactJS? Some advantages of using ReactJS include: Virtual DOM for efficient rendering and updates. Component-based architecture for reusability and maintainability. One-way data flow for predictable state management. Rich ecosystem with a large number of libraries and community support. 3.What is JSX? JSX (JavaScript XML) is a syntax extension used by ReactJS to describe the structure and appearance of UI components. It allows developers to write HTML-like code in JavaScript. 4. What is a component in ReactJS? A component in ReactJS is a reusable and independent piece of UI that encapsulates the logic and presentation of a specific part of the user interface. Components can be composed together to build complex UIs. 5. What is the difference between functional and class components in ReactJS? Functional components are simple JavaScript functions that accept props as input and return JSX as output. They are easier to read, test, and optimize. Class components are JavaScript classes that extend the React.Component class. They have additional features like lifecycle methods and local state. 6. What are hooks in ReactJS? Hooks are functions introduced in React 16.8 that allow developers to use state and other React features in functional components. They provide a way to reuse stateful logic between components. 7. Explain the concept of state in ReactJS. State is an object that holds data specific to a component. It represents the mutable part of the component’s data and can be updated using the setState() method. When state changes, React re-renders the component to reflect the new state. 8. What is the significance of keys in ReactJS lists? Keys are used in ReactJS lists to help identify each item in the list uniquely. They improve performance and help React efficiently update and reorder the list when needed. 9. What is the purpose of the useEffect hook? The useEffect hook is used to perform side effects in functional components. It allows you to manage lifecycle events, such as fetching data, subscribing to events, or manually manipulating the DOM. 10. What is the difference between controlled and uncontrolled components? Controlled components are those in which the component’s state is controlled by React. The component receives its current value via props and notifies changes using callback functions. Uncontrolled components, on the other hand, manage their own state internally using refs and are controlled by the DOM.

1. Top 20 ReactJS Interview Questions and Answers in 2023
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Here are 20 ReactJS interview questions along with their answers:
1. What is ReactJS?
ReactJS is a popular JavaScript library used for building user interfaces. It allows developers to
create reusable UI components and efficiently update the UI in response to changes in data.
2. What are the advantages of using ReactJS?
Some advantages of using ReactJS include:
Virtual DOM for efficient rendering and updates.
Component-based architecture for reusability and maintainability.
One-way data flow for predictable state management.
Rich ecosystem with a large number of libraries and community support.
3.What is JSX?
JSX (JavaScript XML) is a syntax extension used by ReactJS to describe the structure and
appearance of UI components. It allows developers to write HTML-like code in JavaScript.
4. What is a component in ReactJS?
A component in ReactJS is a reusable and independent piece of UI that encapsulates the logic
and presentation of a specific part of the user interface. Components can be composed
together to build complex UIs.

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5. What is the difference between functional and class components in ReactJS?
Functional components are simple JavaScript functions that accept props as input and
return JSX as output. They are easier to read, test, and optimize. Class components are
JavaScript classes that extend the React.Component class. They have additional features
like lifecycle methods and local state.
6. What are hooks in ReactJS?
Hooks are functions introduced in React 16.8 that allow developers to use state and other
React features in functional components. They provide a way to reuse stateful logic
between components.
7. Explain the concept of state in ReactJS.
State is an object that holds data specific to a component. It represents the mutable part
of the component’s data and can be updated using the setState() method. When state
changes, React re-renders the component to reflect the new state.
8. What is the significance of keys in ReactJS lists?
Keys are used in ReactJS lists to help identify each item in the list uniquely. They improve
performance and help React efficiently update and reorder the list when needed.

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9. What is the purpose of the useEffect hook?
The useEffect hook is used to perform side effects in functional components. It allows you
to manage lifecycle events, such as fetching data, subscribing to events, or manually
manipulating the DOM.
10. What is the difference between controlled and uncontrolled components?
Controlled components are those in which the component’s state is controlled by React.
The component receives its current value via props and notifies changes using callback
functions. Uncontrolled components, on the other hand, manage their own state
internally using refs and are controlled by the DOM.
11. What is the role of the setState() method?
The setState() method is used to update the state of a component. It takes an object or a
function as an argument and schedules a re-render of the component with the updated
state.
12. What are the lifecycle methods in ReactJS?
Lifecycle methods are special methods provided by React that allow you to hook into
different stages of a component’s life. Some commonly used lifecycle methods include
componentDidMount(), componentDidUpdate(), and componentWillUnmount().

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13. What is the significance of the render() method in ReactJS?
The render() method is a required method in React components. It returns the JSX
representation of the component’s UI. It is responsible for rendering the component and
its children.
14. What is the purpose of React Router?
React Router is a popular library used for routing in React applications. It allows
developers to define routes and navigate between different components or pages based
on the URL.
15. How can you optimize performance in ReactJS?
To optimize performance in ReactJS, you can implement code splitting and use
React.memo for memoization of components.
16. What is ReactJS and how does it differ from other JavaScript frameworks?
ReactJS is a JavaScript library used for building user interfaces. It differs from other
JavaScript frameworks by using a virtual DOM for efficient rendering, promoting
component-based architecture, and providing a one-way data flow for predictable state
management.

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17. What are the key features of ReactJS?
Some key features of ReactJS include:
Virtual DOM: React uses a virtual representation of the actual DOM for efficient updates
and rendering.
Component-Based Architecture: React encourages building reusable UI components for
easier development and maintenance.
One-Way Data Flow: React follows a unidirectional data flow, making it easier to manage
and track changes in the application state.
JSX: React uses JSX, a syntax extension that allows mixing HTML-like code within JavaScript.
React Hooks: Hooks are functions that allow using state and other React features in
functional components.
18. What is the difference between state and props in ReactJS?
State is used for managing internal component data that can change over time. It is
mutable and can be updated using the setState() method. Props, on the other hand, are
read-only and passed from parent components to child components. They are used to pass
data and behavior down the component hierarchy.

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19. What are React Hooks and why are they used?
React Hooks are functions introduced in React 16.8 that allow using state and other React
features in functional components. They provide a way to reuse stateful logic, such as
managing component state or handling side effects, without using class components.
Hooks like useState and useEffect are commonly used in React applications.
20. How does React Router work in ReactJS?
React Router is a popular routing library for React applications. It allows developers to
define different routes and their corresponding components. When a user navigates to a
specific URL, React Router matches the URL with the defined routes and renders the
corresponding component. It also provides features like nested routing, dynamic routing,
and URL parameters to handle complex routing scenarios in React applications.