Introduction
React JS is a popular JavaScript library for building user interfaces. It was created by Facebook and is now maintained by Meta and the open-source community. React JS allows you to create reusable UI components that can display dynamic data. React JS uses a virtual DOM to update the browser’s DOM efficiently and avoid unnecessary re-rendering. React JS is also component-based, which means you can compose complex applications from simple building blocks.
React JS is widely used for developing web and mobile applications, such as Facebook, Instagram, WhatsApp, Netflix, Airbnb, and more.
React JS Interview Questions & Answers
What is React.js, and how does it differ from other JavaScript frameworks?
What is React.js, and how does it differ from other JavaScript frameworks?
What are the key features of React?
Virtual DOM, Components, JSX, One-way data binding, and React Native.
Explain JSX in React.
JSX (JavaScript XML) is a syntax extension for JavaScript used in React to describe the structure of UI components in a more declarative way.
What is a React Component?
A React component is a reusable building block for UI elements, representing parts of a user interface.
What are the differences between functional and class components?
Functional components are simpler, use function syntax, and are stateless. Class components can have state and lifecycle methods.
What is state in React?
State is a built-in object used to store and manage data that can change over time in a React component.
Explain the concept of props in React.
Props (short for properties) are used for passing data from parent to child components.
What is the Virtual DOM, and how does it improve performance?
The Virtual DOM is an in-memory representation of the actual DOM. React uses it to optimize updates by minimizing direct manipulation of the real DOM.
What is the difference between controlled and uncontrolled components in React?
Controlled components are tied to state and are updated via props, while uncontrolled components store their data in the DOM.
What are React Hooks, and why are they used?
React Hooks are functions that allow functional components to manage state and side effects, replacing class component features.
Explain the useEffect Hook in React.
useEffect is used to perform side effects in functional components, such as data fetching or DOM manipulation.
How can you conditionally render elements in React?
You can use conditional statements like if or the ternary operator (? 🙂 to conditionally render elements.
What is Redux, and how does it work with React?
Redux is a state management library that helps manage the application’s global state. It can be used with React to share data between components.
What is the purpose of Redux Thunk?
Redux Thunk is middleware for handling asynchronous actions in Redux, allowing you to dispatch functions instead of just objects.
How does React Router work, and what are its main components?
React Router is used for client-side routing in React applications. It uses components like <Route> and <Link> to define and navigate between routes.
What is PureComponent, and when would you use it?
PureComponent is a class component that performs a shallow comparison of props and state to optimize rendering in certain cases.
How can you optimize performance in React applications?
Use PureComponent or memoization, minimize unnecessary re-renders, use lazy loading, and employ code splitting.
What is the purpose of keys in React lists?
Keys are used to help React identify which items have changed, been added, or been removed in a list, optimizing updates.
What are portals in React?
Portals allow rendering a component’s children into a different DOM hierarchy outside the parent component.
Explain the concept of HOCs (Higher-Order Components) in React.
HOCs are functions that take a component and return a new component with enhanced functionality.
What is React Fiber?
React Fiber is the ongoing rewrite of the React core algorithm to improve the performance of rendering and enable features like asynchronous rendering.
What are fragments in React, and why are they used?
Fragments are a way to group multiple elements without adding an extra DOM element. They help avoid unnecessary div wrappers.
What are PropTypes in React, and why are they used?
PropTypes are used to specify the type of props a component should receive. They help with debugging and catching runtime errors.
What is the purpose of the key prop when rendering lists of elements?
The key prop is used to help React identify which items in a list have changed, been added, or been removed efficiently.
Explain the concept of state lifting in React.
State lifting is a pattern in React where you move the state of multiple components to a common ancestor to share data between them.
What are controlled and uncontrolled components in form handling?
Controlled components have form elements where their value is controlled by React state, while uncontrolled components manage their state in the DOM.
What is the significance of the “shouldComponentUpdate” lifecycle method?
shouldComponentUpdate” allows you to optimize rendering by deciding whether a component should re-render when props or state change.
What is React context, and when would you use it?
React context allows you to share data between components without manually passing props through intermediate components. It’s useful for global data.
How do you handle routing in a React application without using React Router?
You can use the History API or build your custom routing solution with event listeners and state management.
What is the purpose of the “keyExtractor” function in a React Native FlatList?
“keyExtractor” is used to specify how to extract unique keys for items in a FlatList to optimize rendering.
Explain the use of the “dangerouslySetInnerHTML” prop in React.
“dangerouslySetInnerHTML” is used to inject HTML into a component, but it should be used cautiously because it can lead to security vulnerabilities.
How do you handle forms in React?
You can use controlled components, uncontrolled components, or form libraries like Formik or React Hook Form.
What is the significance of the “componentDidMount” and “componentWillUnmount” lifecycle methods?
“componentDidMount” is called after a component is mounted in the DOM, and “componentWillUnmount” is called before a component is removed from the DOM.
How do you pass data from child to parent components in React?
You can pass a callback function as a prop to the child, allowing the child to communicate with the parent.
What are React keys, and why are they necessary in lists?
React keys are unique identifiers for elements in lists, enabling React to efficiently update and re-render lists.
What is the purpose of the “componentDidUpdate” lifecycle method?
“componentDidUpdate” is called after a component updates in the DOM and can be used for side effects or updating state based on props or state changes.
Explain the difference between “componentWillReceiveProps” and “getDerivedStateFromProps” lifecycle methods.
“componentWillReceiveProps” is deprecated and used for side effects. “getDerivedStateFromProps” is used to derive state from props and is static.
What is the React DevTools extension, and how can it help developers?
React DevTools is a browser extension that allows developers to inspect and debug React component hierarchies and their state.
How does React handle security concerns like XSS (Cross-Site Scripting)?
React uses techniques like escaping and “dangerouslySetInnerHTML” to mitigate XSS vulnerabilities.
What is the React StrictMode, and why is it beneficial?
React StrictMode is a tool that highlights potential problems in your application during development, helping you catch and fix issues early.
Explain the concept of memoization in React.
Memoization is a technique used to optimize expensive computations or function calls by caching their results based on input parameters.
What are the limitations of using React Native for mobile app development?
Limitations include platform-specific code, limited third-party library support, and performance differences between native and React Native apps.
How can you optimize the performance of a React Native app?
You can optimize performance by using native modules, avoiding unnecessary re-renders, and profiling and optimizing JavaScript code.
What is the significance of the “useEffect” dependency array?
The dependency array in the “useEffect” hook specifies which values should trigger the effect to re-run when they change.
How does Redux differ from the React context API for state management?
Redux is a standalone library for state management, while the React context API is built into React and used for sharing data between components.
What are React Portals, and when would you use them?
React Portals allow rendering components into a different DOM tree. They are useful for scenarios like modal dialogs or tooltips.
Explain the concept of “render props” in React.
Render props is a technique where a component takes a function as a prop and calls it with data that the component wants to share.
What is the significance of “useState” in functional components?
“useState” is a hook that allows functional components to manage local state.
What are the advantages of using Redux for state management in large-scale applications?
Redux provides a centralized store, a predictable state container, and powerful debugging tools, making it suitable for complex applications.
What is the purpose of the “useReducer” hook in React?
useReducer” is a hook used for managing complex state logic by dispatching actions to update state.
How can you optimize the performance of a React application?
Performance optimization techniques include using shouldComponentUpdate, memoization, code splitting, lazy loading, and minimizing re-renders.
What is the Context API, and how does it work in React?
The Context API provides a way to share data between components without manually passing props through the component tree.
Explain the concept of server-side rendering (SSR) in React.
Server-side rendering is the process of rendering React components on the server and sending HTML to the client, improving initial page load times.
What are hooks in React, and how do they differ from class components?
Hooks are functions that allow functional components to manage state and side effects. They replace class component features like state and lifecycle methods.
What is the purpose of the “useContext” hook in React?
“useContext” is a hook used to access the current context value within a functional component.
How can you prevent unnecessary re-renders in functional components?
You can use the “memo” function, “PureComponent,” or the “useMemo” and “useCallback” hooks to prevent unnecessary re-renders.
Explain the concept of code splitting in React.
Code splitting is the process of splitting your application’s JavaScript bundle into smaller files to reduce initial load times.
What is the significance of the “React.Fragment” component?
“React.Fragment” is a built-in component that allows you to group multiple elements without introducing an extra DOM element.
What is the purpose of the “React.memo” function?
“React.memo” is a higher-order component that memoizes a functional component, preventing unnecessary re-renders.
How does React handle prop drilling, and what are the solutions to avoid it?
Prop drilling occurs when props are passed through multiple levels of components. Solutions include using context or state management libraries like Redux.
What is the significance of the “useLayoutEffect” hook in React?
“useLayoutEffect” is similar to “useEffect” but fires synchronously after all DOM mutations. It’s useful for interacting with the DOM before painting.
How does React handle forms with multiple input fields that need to be controlled?
You can manage multiple input fields by maintaining separate state variables for each field or using form libraries like Formik.
Explain the concept of lazy loading in React.
Lazy loading involves loading components or assets only when they are needed, improving the initial load time of your application.
What is the significance of the “React.StrictMode” component, and how does it affect your application?
“React.StrictMode” is a wrapper component that helps catch and identify potential problems in your application during development.
What is Redux-Saga, and how does it work with Redux for side effects?
Redux-Saga is a middleware library used for handling side effects in Redux by managing asynchronous actions using generators.
How can you optimize the performance of a React Native application?
Performance optimization techniques for React Native include using native modules, optimizing UI rendering, and profiling for bottlenecks.
What is the purpose of the “forwardRef” function in React?
“forwardRef” allows you to pass refs through components, making it possible to access the DOM or React elements of child components.
How does the “useMemo” hook work in React, and when would you use it?
“useMemo” memoizes the result of a function and returns the cached result unless the dependencies change. It’s used to optimize expensive calculations.
What is the significance of the “useEffect” hook’s cleanup function?
The cleanup function returned by “useEffect” is used to clean up resources or cancel asynchronous operations when a component unmounts or re-renders.
Explain the concept of code splitting with React Router.
Code splitting with React Router involves dynamically loading and rendering different parts of your application based on the route, improving performance.
How can you conditionally apply styles to React components?
You can conditionally apply styles using inline styles, CSS classes, or libraries like styled-components.
What is the purpose of the “React.lazy” function, and how does it work?
“React.lazy” is used for lazy loading components. It dynamically imports a component when it’s needed, reducing initial load times.
What is the significance of the “useCallback” hook in React?
“useCallback” memoizes a callback function and returns the cached version unless the dependencies change. It’s used to optimize rendering.
How can you handle side effects in functional components without using “componentDidMount” or “componentDidUpdate”?
You can use the “useEffect” hook to handle side effects in functional components.
What is the purpose of the “useImperativeHandle” hook in React?
“useImperativeHandle” allows you to customize the instance value that is exposed when using “React.forwardRef.”
Explain the concept of “controlled” and “uncontrolled” components in React forms.
Controlled components have their state managed by React, while uncontrolled components store their state in the DOM.
What are the advantages of using TypeScript with React?
TypeScript provides static typing, better tooling, and improved code quality and documentation in React projects.
How do you prevent the default behavior of an event in React?
You can use the “preventDefault” method on the event object to prevent the default behavior of an event.
What is the purpose of the “useHistory” hook in React Router?
“useHistory” is a hook in React Router that provides access to the browser’s navigation history, allowing you to programmatically navigate.
How does server-side rendering (SSR) improve the performance of a React application?
SSR improves performance by rendering the initial HTML on the server, reducing the time it takes for the page to become interactive.
What are React keys, and why are they essential in lists?
React keys are used to identify and track individual items in lists, ensuring efficient updates and rendering.
Explain the concept of context in React and how it works.
Context allows you to share data between components without manually passing props through the component tree. It uses a Provider and Consumer pattern.
What are hooks in React, and why were they introduced?
Hooks are functions that allow functional components to manage state and side effects. They were introduced to simplify state management and lifecycle logic.
How can you handle asynchronous operations in React components?
You can use the “useEffect” hook for side effects involving asynchronous operations like data fetching.
What is the significance of the “dangerouslySetInnerHTML” prop in React?
“dangerouslySetInnerHTML” is used to insert HTML directly into a component, but it should be used carefully due to security risks.
What is the difference between “state” and “props” in React?
State is used to manage data that can change within a component, while props are used to pass data from a parent to a child component.
Explain the concept of lazy loading in React and how it improves performance.
Lazy loading involves loading and rendering components or assets only when they are needed, reducing the initial load time and improving performance.
How does React handle forms, and what are controlled components?
React handles forms by using controlled components, where form elements’ values are controlled by React state.
What is the significance of the “shouldComponentUpdate” method in React components?
“shouldComponentUpdate” allows you to optimize rendering by determining whether a component should re-render when its props or state change.
What are hooks, and why are they important in React?
Hooks are functions that allow functional components to manage state and side effects. They are important for simplifying component logic.
How can you handle events in React, and what is event delegation?
You can handle events in React by using event handlers like “onClick.” Event delegation is a technique where a single event handler is attached to a common ancestor element to handle events for multiple children.
Explain the concept of synthetic events in React.
Synthetic events are wrappers around native DOM events, providing a consistent API for handling events across different browsers.
What is the purpose of the “useRef” hook in React?
“useRef” is used to access and interact with a DOM element or to persist values across renders without causing re-renders.
How can you dynamically render components in React?
You can dynamically render components by conditionally rendering them using conditional statements or mapping through an array of components.
What is the significance of the “componentDidCatch” lifecycle method in React?
“componentDidCatch” is used for error handling and is called when an error occurs in a component’s child components.
How can you pass data from a child component to a parent component in React?
You can pass data from a child to a parent component by defining a callback function in the parent component and passing it as a prop to the child.
What is the significance of the “key” prop when rendering lists in React?
The “key” prop is used to identify and differentiate elements in a list, ensuring efficient updates and rendering.
How does React handle security concerns like Cross-Site Scripting (XSS)?
React provides built-in mechanisms like escaping and the “dangerouslySetInnerHTML” prop to mitigate XSS vulnerabilities.
What is the purpose of the “useLayoutEffect” hook in React, and how does it differ from “useEffect”?
– “useLayoutEffect” is similar to “useEffect” but runs synchronously after all DOM mutations. It’s used for interactions with the DOM before rendering.
