React Interview Questions & Answers
What is React.js?
React.js is a JavaScript library used for building user interfaces. It allows developers tocreate reusable UI components and efficiently update the UI when the data changes.
What are the key features of React.js?
React.js offers features like virtual DOM, component-based architecture, one-way data flow, JSX syntax, and a rich ecosystem of libraries and tools.
What is JSX?
JSX (JavaScript XML) is an extension to JavaScript that allows you to write HTML-like
syntax within JavaScript code. It is used to describe the structure and appearance of React components.
What is the significance of the virtual DOM in React.js?
The virtual DOM is a lightweight copy of the actual DOM. React uses the virtual DOM to optimize and speed up the process of updating the real DOM by comparing the current virtual DOM with the previous one.
What is the difference between a functional component and a class component in React.js?
Functional components are stateless and are typically written as plain JavaScript functions. They are simpler and easier to test. Class components, on the other hand, have a state, can use lifecycle methods, and are written as ES6 classes.
What is the purpose of the constructor in a React component?
The constructor is used to initialize the state and bind event handlers in a class component. It is called before the component is mounted.
What is state in React.js?
State is an object that holds data and determines how a component renders and behaves. It is private and fully controlled by the component itself.
What is the difference between state and props in React.js?
State is managed within a component and can be changed, while props are passed to a component from its parent and cannot be modified directly by the component receiving them.
What is a controlled component?
A controlled component is a component where the form data is handled by React components. The React component that renders the form also controls what happens in that form on subsequent user input.
What are React lifecycle methods?
React lifecycle methods are special methods that are called at specific points in a component's lifecycle. These methods include componentDidMount, componentDidUpdate, componentWillUnmount, and many others.
What is the significance of the render() method in React.js?
The render() method in React.js is responsible for returning the JSX that represents the structure and appearance of a component. It gets called whenever the component updates.
What is the purpose of keys in React lists?
Keys are used to give a unique identity to each element in a list of components. They help React efficiently update and re-render the components by identifying which items have changed, been added, or removed.
What is the context in React.js?
Context provides a way to pass data through the component tree without having to pass props manually at every level. It is used for sharing data that can be considered "global" for a tree of React components.
What are higher-order components (HOCs) in React.js?
Higher-order components are functions that take a component and return a new component with additional functionality. They are used for code reuse, abstraction, and sharing common logic between components.
What are React hooks?
Hooks are functions that allow you to use state and other React features without writing a class. They were introduced in React 16.8 to provide a simpler and more readable way to write React components.
What are the basic rules of hooks?
Hooks have some rules: They must be used only at the top level of a function component, hooks mustbe called in the same order every time the component renders, and they cannot be called conditionally.
What is the useState hook used for?
The useState hook is used to add state to functional components. It returns a stateful value and a function to update that value. By calling the update function, the component can trigger a re-render with the updated state.
What is the useEffect hook used for?
The useEffect hook is used to perform side effects in functional components. It allows you to run code after the component has rendered and handle actions such as data fetching, subscriptions, or manually updating the DOM.
What is the difference between useCallback and useMemo hooks?
useCallback is used to memoize functions, preventing unnecessary re-creation of functions on re-renders. useMemo is used to memoize values, caching the result of expensive calculations and avoiding recomputation.
What is React Router?
React Router is a popular library for handling routing in React applications. It allows you to define different routes, render components based on the current URL, and navigate between different views in a single-page application.
What is Redux?
Redux is a predictable state container for JavaScript applications. It provides a centralized store to manage application state and uses actions and reducers to modify and update that state in a predictable manner.
What is the purpose of actions in Redux?
Actions in Redux are plain JavaScript objects that describe an event or user interaction. They are dispatched to the store and trigger the corresponding reducer to update the state based on the action's type and payload.
What are reducers in Redux?
Reducers in Redux are pure functions that specify how the application's state changes in response to actions. They take the current state and an action as input and return a new state based on that action.
What is the connect function in Redux?
The connect function is used to connect a React component to the Redux store. It provides the component with access to the store's state and actions, allowing it to subscribe to changes and dispatch actions.
What is the purpose of middleware in Redux?
Middleware in Redux provides a way to intercept and modify actions before they reach the reducers. It can be used for handling asynchronous actions, logging, and other tasks that need to be done outside the normal action flow.
What is Redux Thunk?
Redux Thunk is a middleware for Redux that allows you to write action creators that return functions instead of plain action objects. This enables handling of asynchronous actions, such as API calls, inside action creators.
What is React Native?
React Native is a framework for building native mobile applications using React. It allows developers to write mobile apps using JavaScript and leverage the power of React to create reusable UI components.
What is the difference between React and React Native?
React is a JavaScript library for building user interfaces, primarily for web applications, while React Native is a framework for building native mobile applications. React Native uses native components and APIs specific to each platform.
What are React Native components?
React Native components are similar to React components but are built specifically for mobile app development. They include components for handling user input, displaying data, navigating between screens, and more.
What is the purpose of StyleSheet in React Native?
StyleSheet is a built-in component in React Native that allows you to define styles for your components. It provides a way to write styles using JavaScript objects or create reusable style constants.
What is the difference between state and props in React Native?
The difference between state and props in React Native is the same as in React.js. State is managed within a component and can be changed, while propsare passed to a component from its parent and cannot be modified directly by the component receiving them.
What is the purpose of AsyncStorage in React Native?
AsyncStorage is a simple, asynchronous, persistent key-value storage system provided by React Native. It allows you to store data on the device's disk and retrieve it later, making it useful for caching data or storing user preferences.
What is the purpose of the Expo framework in React Native?
Expo is a set of tools, libraries, and services built on top of React Native. It provides a simplified development workflow, pre-configured native modules, and access to device features, allowing developers to build and deploy React Native apps faster.
What is the purpose of the shouldComponentUpdate() method?
The shouldComponentUpdate() method is a lifecycle method in React that determines whether a component should re-render or not. By implementing this method and returning false under certain conditions, you can optimize the performance of your application.
What is the React DevTools?
React DevTools is a browser extension that allows you to inspect and debug React component hierarchies. It provides a set of tools for inspecting components, examining props and state, and profiling performance.
What is the purpose of the key prop in React?
The key prop is used to give a unique identity to each element in a list of components. It helps React efficiently update and re-render the components by identifying which items have changed, been added, or removed.
What are the different ways to style components in React?
There are multiple ways to style components in React, including inline styles, using CSS classes with className, CSS-in-JS libraries like styled-components, and using preprocessor-based solutions like Sass or Less.
What is the purpose of React Fragments?
React Fragments allow you to group multiple elements without adding an extra node to the DOM. They are useful when you need to return multiple elements from a component's render method without introducing unnecessary wrapping elements.
What are controlled components in React forms?
Controlled components are form elements whose values are controlled by React state. The state is updated whenever the user interacts with the form, and the input values are set explicitly using React state, allowing for more control and validation.