Reactjs Interview Question And Answers, ReactJS online training

React.js, commonly referred to as React, is a JavaScript library developed and maintained by Facebook. It is used for building user interfaces, particularly for creating single-page applications where the content is dynamically updated and rendered efficiently. React was open-sourced in 2013 and has since gained widespread adoption in the web development community. Lets see some of the important Reactjs interview question and answers.

1. What is React.js?

Answer: React.js is a JavaScript library for building user interfaces. It allows developers to create reusable UI components and manage the state of the application efficiently.

2. What is JSX?

Answer: JSX (JavaScript XML) is a syntax extension for JavaScript recommended by React. It allows you to write HTML elements and components in a syntax similar to XML or HTML.

3. Explain the Virtual DOM in React.

Answer: The Virtual DOM is a programming concept where an ideal, or “virtual,” representation of the user interface is kept in memory. React uses a virtual DOM to improve the efficiency of updating the actual DOM by minimizing direct manipulations and updating only the necessary parts.

4. What is the significance of state in React?

Answer: The state is a JavaScript object that stores component-specific data. It allows components to manage and maintain their data, making them dynamic and responsive to user inputs.

5. Explain the difference between props and state.

Answer: Props (short for properties) are used to pass data from parent components to child components, while state is used to manage the internal data of a component. Props are immutable, and changes to props should be handled by the parent component, whereas state is mutable and can be changed within the component.

6. What are React Hooks?

Answer: React Hooks are functions that enable functional components to use state and lifecycle features that were previously only available in class components. Examples include useState for managing state and useEffect for handling side effects.

7. Explain the concept of controlled components.

Answer: In React, a controlled component is an input element (like text input or textarea) whose value is controlled by React state. This means that the component’s state is the single source of truth for the input value, and any changes to the input value are handled by the React state.

8. What is the purpose of the `key` attribute in React lists?

Answer: The key attribute is used to give a unique identity to each element in a list of React elements. It helps React efficiently update and re-render the components in the list by providing a stable identity to each item.

9. Explain the significance of `componentDidMount` and `componentDidUpdate` lifecycle methods.

Answer: componentDidMount is invoked immediately after a component is mounted to the DOM. It is commonly used to initiate network requests or set up subscriptions. componentDidUpdate is invoked after a component is updated, and it is useful for performing actions after a component re-renders due to changes in state or props.

10. What is React Router?

Answer: React Router is a library for handling navigation in React applications. It enables the creation of single-page applications with navigation and views that are dynamically updated as the URL changes. It provides components like `BrowserRouter` and `Link` for creating a navigation system.