Top 10 React Js Interview Questions and Answers

1. What is React.js?

   Answer: React.js is an open-source JavaScript library developed by Facebook for building user interfaces or UI components, particularly for single-page applications where the UI needs to be updated dynamically as the user interacts with it.

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 that looks similar to XML or HTML. JSX gets compiled to JavaScript and does not directly represent HTML. It provides a concise and familiar syntax for defining React elements.

3. What are components in React?

   Answer: Components are the building blocks of a React application. They are reusable and independent pieces of code that encapsulate a part of the UI. Components can be either functional (stateless) or class-based (stateful). They promote reusability, maintainability, and a modular structure in React applications.

4. What is the difference between state and props in React?

   Answer: Props (short for properties) are inputs to a React component that are passed from its parent component, while state is an internal data storage mechanism that allows a component to keep track of changes. Props are immutable, and changes to props in the parent component trigger updates in the child component, whereas state changes trigger re-rendering within the component itself.

5. Explain the concept of virtual DOM in React.

   Answer: The virtual DOM is a programming concept where an in-memory representation of the actual DOM elements is maintained. React uses the virtual DOM to optimize the updating process. When state changes occur, React creates a new virtual DOM tree, compares it with the previous one, and then updates only the parts of the actual DOM that have changed. This minimizes the number of direct manipulations on the real DOM, improving performance.

6. What is the significance of keys in React?

   Answer: Keys are used in React to help identify which items in a list have changed, been added, or been removed. They should be unique among sibling elements. React uses keys to efficiently update the DOM by minimizing re-rendering and reordering of components.

7. How does React handle data binding?

   Answer: React follows a unidirectional data flow, meaning data changes in the parent components are passed down to child components via props. To handle user input and other interactive features, React uses synthetic events similar to regular JavaScript events but with cross-browser compatibility. State is then updated, triggering a re-render of the component.

8. What is the significance of `setState` in React?

   Answer: `setState` is a method provided by React to update a component's local state. When `setState` is called, React schedules an asynchronous update, merging the provided object into the current state. This triggers a re-render of the component and its children. It's important to note that state updates in React are asynchronous to ensure performance.

9. Explain the purpose of React Router.

   Answer: React Router is a library for handling navigation in a React application. It enables the creation of single-page applications with navigation between different views or components. React Router provides a `<BrowserRouter>` component for client-side routing and allows developers to define routes, which map specific URLs to corresponding components.

10. What are React Hooks?

    Answer: React Hooks are functions that allow functional components to manage state, side effects, and other React features that were previously only available in class components. Examples include `useState` for managing state and `useEffect` for handling side effects. Hooks were introduced in React 16.8 to make it easier to use state and other React features in functional components.

These questions cover a range of fundamental concepts in React.js and can be a good starting point for interview preparation.