ReactJS Interview Questions

What is ReactJS?

ReactJS is a popular JavaScript library used for building user interfaces in web applications. Developed by Facebook, it allows developers to create reusable UI components that efficiently update and render data without the need for manually refreshing the entire page. ReactJS follows a component-based architecture for building interactive interfaces.

Explain the Virtual DOM in ReactJS.

The Virtual DOM in ReactJS is a lightweight copy of the actual DOM that React uses to efficiently update the user interface. When changes are made, React compares the Virtual DOM with the real DOM to identify the minimal number of updates needed, reducing the overall re-rendering process and improving performance.

What are the key features of ReactJS?

Key features of ReactJS include a virtual DOM for efficient updates, component-based architecture for reusability, JSX syntax for easy UI development, one-way data flow for predictable state management, and a rich ecosystem of libraries and tools. React also allows server-side rendering for improved performance and SEO.

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How does ReactJS differ from other JavaScript frameworks?

ReactJS differs from other JavaScript frameworks in several ways. It utilizes a virtual DOM for better performance, allows for reusable components for a more modular code structure, and follows a one-way data binding approach. Additionally, React's syntax incorporates JSX, which enables mixing HTML and JavaScript code seamlessly.

What is JSX in ReactJS?

JSX in ReactJS is a syntax extension that allows developers to write HTML-like code directly within their JavaScript code. It makes it easier to create and manipulate UI elements, as JSX gets compiled into regular JavaScript code that the browser can understand.

Explain the difference between state and props in ReactJS.

In ReactJS, props are used to pass data from parent to child components, and they are immutable. State, on the other hand, is used to store data that can be changed within a component, and is mutable. Props are set by the parent components, while state is managed within the component itself.

What are React Hooks and how are they used?

React Hooks are functions that enable functional components to use state and lifecycle features that were previously only available in class components. They allow developers to use state, effects, context, and other React features without the need for class components, simplifying and improving the readability of code.

How does React handle data binding?

React handles data binding using a one-way data flow system. It utilizes the Virtual DOM to efficiently update and re-render components as the data changes. By creating a unidirectional data flow, React ensures that data changes are propagated in a predictable and controlled manner throughout the application.

What is the purpose of the key prop in React lists?

The key prop in React lists is used to uniquely identify elements in a list of components. It helps React identify which items have changed, are added, or are removed from the list. This key prop optimizes performance by allowing React to efficiently update the DOM when the list changes.

Explain the concept of conditional rendering in ReactJS.

Conditional rendering in ReactJS involves displaying different content or components based on certain conditions. This can be achieved by using JavaScript expressions within JSX to conditionally show or hide elements, allowing for dynamic rendering based on user interactions or data changes.

How do you handle forms in ReactJS?

In ReactJS, forms are typically handled by creating controlled components, where form input values are stored in the component's state. Event handlers are used to update the state when form inputs change, and the onSubmit event is used to handle form submission and data processing.

What are Higher-Order Components (HOC) in ReactJS?

Higher-Order Components (HOC) are a pattern in ReactJS where a function takes a component as an argument and returns a new component with enhanced functionality. HOCs allow for code reusability, logic abstraction, and can be used to add features such as authentication, data fetching, or styling to components.

What is the significance of using Fragments in ReactJS?

Fragments in ReactJS are used to group multiple elements together without adding extra nodes to the DOM. This helps in improving performance by avoiding unnecessary div elements while maintaining the structure of components. Fragments are particularly useful when returning adjacent JSX elements from a component.

Explain the component lifecycle methods in ReactJS.

The component lifecycle methods in ReactJS include componentWillMount, componentDidMount, componentWillReceiveProps, shouldComponentUpdate, componentWillUpdate, componentDidUpdate, componentWillUnmount. These methods allow developers to interact with the lifecycle of a React component, enabling them to manage state, perform side effects, and optimize performance.

How do you optimize performance in ReactJS applications?

To optimize performance in ReactJS applications, you can implement techniques such as code splitting, lazy loading components, using shouldComponentUpdate for class components, using useMemo and useCallback hooks, minimizing the use of inline functions, avoiding unnecessary re-renders, and utilizing virtualization for large lists. Optimize state management with libraries like Redux or React Context.

What are the different types of components in ReactJS?

In ReactJS, there are two main types of components: Functional Components and Class Components. Functional Components are simple functions that take props as arguments and return React elements. Class Components are ES6 classes that extend from React.Component, allowing for more features like state and lifecycle methods.

How can you handle events in ReactJS?

In ReactJS, you can handle events by using event handlers such as onClick, onChange, onSubmit, etc. These event handlers are added to JSX elements and are defined as methods in the component class. When the event is triggered, the corresponding method is executed to handle the event.

What are controlled and uncontrolled components in ReactJS?

Controlled components in ReactJS have their state controlled by the parent component through props, while uncontrolled components have their state managed internally using refs. Controlled components provide more control and flexibility over the form data, while uncontrolled components offer simplicity and are useful for managing form inputs with default values.

Explain the process of rendering lists in ReactJS.

To render lists in ReactJS, you can use the map() method to iterate over an array of elements and return a new array of React components. Each component should have a unique key attribute to help React efficiently update the list when it changes.

What is Context API in ReactJS and how is it used?

Context API in ReactJS is a feature that allows you to share data between components without having to pass props down manually at every level. It provides a way to pass data through the component tree without having to pass props down manually at every level. It is commonly used for managing global state in a React application.

What is ReactJS?

ReactJS is a popular JavaScript library used for building user interfaces in web applications. Developed by Facebook, it allows developers to create reusable UI components that efficiently update and render data without the need for manually refreshing the entire page. ReactJS follows a component-based architecture for building interactive interfaces.

ReactJS is a popular JavaScript library for building user interfaces. It was developed by Facebook and released to the public in 2013. React allows developers to create reusable UI components and manage the state of these components efficiently. React uses a declarative approach, making it easier to design simple views for each state in your application and React efficiently updates and renders the right components when your data changes.

One key feature of React is the use of a virtual DOM (Document Object Model). Virtual DOM is a lightweight copy of the actual DOM and allows React to perform updates in an efficient manner. When the state of a component changes, React compares the virtual DOM with the actual DOM to identify the minimal number of changes required to update the view, improving performance and responsiveness of the application.

Here is an example of a simple React component that renders a "Hello, World!" message:

import React from 'react';

class HelloMessage extends React.Component {
    render() {
Hello, World!
; } } export default HelloMessage;

React supports the concept of reusable components, allowing developers to create modular and maintainable UI elements. Components can have their own state and lifecycle methods, making it easier to manage the behavior of the application.

Key Features of ReactJS:

  • Component-Based Architecture: React applications are built using reusable components that encapsulate the UI elements and their behavior.
  • Virtual DOM: React utilizes a virtual DOM to efficiently update and render components without affecting the actual DOM directly.
  • JSX: JSX is a syntax extension for JavaScript that allows developers to write HTML-like code within their JavaScript files, making it easier to create React elements.
  • One-Way Data Binding: React follows a unidirectional data flow, where changes in the state or props of a component trigger re-renders of the affected components.

React has gained widespread adoption in the web development community due to its simplicity, performance, and flexibility. It is often used in combination with other libraries and frameworks, such as Redux for state management and React Router for routing in single-page applications.