React Interview Questions
1.What is React?
React is an open-source JavaScript library for building user interfaces, primarily for single-page applications and complex web applications. It was developed by Facebook and is commonly used for creating interactive and dynamic UI components.
2.Explain the key features of React
React has several key features, including:
- Virtual DOM: React uses a virtual representation of the actual DOM for efficient updates.
- Component-Based Architecture: It encourages building UIs using reusable components.
- JSX: A syntax extension that allows writing HTML within JavaScript code.
- Unidirectional Data Flow: Data flows in one direction, making it predictable.
3.What is JSX in React?
JSX (JavaScript XML) is a syntax extension in React that allows you to write HTML-like code within JavaScript. It is transpiled into JavaScript for the browser to understand.
4.Explain the difference between React and React Native
React is a library for building web applications, while React Native is a framework for building mobile applications for iOS and Android. React Native uses the same component-based approach as React but targets mobile platforms
5.What is a component in React?
In React, a component is a reusable building block for UI elements. Components can be class-based or functional and encapsulate their logic and rendering.
6.What is the difference between a functional component and a class component in React?
Functional components are simple JavaScript functions that receive props as arguments and return JSX. Class components are ES6 classes that extend React.Component and can manage state and lifecycle methods.
7.What are props in React?
Props (short for properties) are a mechanism for passing data from a parent component to a child component. They are read-only and help make components reusable and dynamic.
8.Explain state in React
State is a built-in object in React components used for managing component-specific data that can change over time. It is mutable and can be updated using setState.
9.What is the difference between state and props in React?
State is used to manage component-specific data that can change over time and is mutable, while props are used for passing data from parent to child components and are read-only.
10.How do you update the state of a React component?
You can update the state of a React component using the setState method. It takes an object that represents the new state or a function that updates the state based on the previous state.
11.What are React hooks, and how do they work?
React hooks are functions that allow functional components to use state and other React features. For example, useState is used to add state to functional components, and useEffect is used for side effects.
12.Explain the purpose of the useEffect hook in React
The useEffect hook is used for managing side effects in functional components, such as data fetching, DOM manipulation, and more. It is called after rendering and can mimic component lifecycle methods.
13.What is the virtual DOM in React, and why is it important?
The virtual DOM is a lightweight copy of the actual DOM. React uses it to perform efficient updates by comparing the virtual DOM with the real DOM and making minimal changes, reducing the rendering cost.
14.What is the significance of keys in React when rendering lists of elements?
Keys are used to give React a way to identify which elements in a list have changed, been added, or removed. It helps React efficiently update the UI when rendering lists.
15.What is the purpose of refs in React?
Refs provide a way to access and interact with a DOM element directly. They are often used to integrate React with non-React libraries and for managing focus and text selection.
16.What is the difference between controlled and uncontrolled components in React forms?
Controlled components are React components where form data is controlled by React state, while uncontrolled components store form data in the DOM, and React doesn’t control it.
17.Explain the concept of conditional rendering in React
Conditional rendering is the practice of rendering components or elements based on certain conditions. You can use if statements, ternary operators, or logical expressions to conditionally render elements.
18.What is prop drilling in React, and how can you avoid it?
Prop drilling occurs when you pass props through multiple levels of nested components. You can avoid it by using state management tools like Context API or libraries like Redux.
19.What is Redux, and why is it used in React applications?
Redux is a state management library for JavaScript applications, often used with React. It provides a predictable state container that makes it easier to manage and share data across components.
20.What is the React Context API, and how does it work?
The React Context API is a way to pass data through the component tree without having to pass props manually at every level. It provides a Provider and Consumer mechanism for sharing data.
Experience the best React training in Hyderabad at The LAMP Institute. Take advantage of reasonable costs, knowledgeable instructors with fifteen years of expertise, complete placement support, practice interviews, resume writing, practical instruction, and internship opportunities.”
21.Explain the concept of high-order components (HOC) in React
Higher-order components are functions that take a component and return a new component with enhanced functionality. They are used for code reuse and adding additional props or behavior to components.
22.What are React hooks and why were they introduced?
React hooks were introduced to allow functional components to have state and lifecycle features without the need for class components. They make it easier to reuse stateful logic and side effects.
23.What is lazy loading in React, and why is it beneficial?
Lazy loading is a technique where components or resources are loaded only when they are needed. It can improve application performance by reducing the initial bundle size.
24.Explain the concept of code splitting in React
Code splitting is the practice of breaking the application’s code into smaller chunks to load only the required code for a particular route or feature, improving performance.
25.What is the purpose of the React Router library, and how do you use it?
React Router is a library for handling client-side routing in React applications. It allows you to define routes and their associated components, enabling navigation within a single-page application.
26.What is the difference between server-side rendering (SSR) and client-side rendering (CSR) in React?
SSR renders a web page on the server and sends the fully rendered HTML to the client, while CSR renders the page on the client side using JavaScript. SSR is beneficial for SEO and initial page load performance.
27.Explain the concept of error boundaries in React
Error boundaries are components that catch JavaScript errors during rendering, in lifecycle methods, and during the constructor of the whole component tree. They help prevent the entire UI from crashing.
28.What is the purpose of PropTypes in React, and how do you use them?
PropTypes are a type-checking mechanism for props in React components. They help catch bugs early and ensure that the correct data types are passed as props. PropTypes can be defined using the prop-types library.
29.What is the significance of CSS-in-JS libraries like styled-components in React?
CSS-in-JS libraries like styled-components allow you to write CSS styles directly in your JavaScript code. This approach provides scoped styles, dynamic styling, and better component encapsulation.
30.How can you optimize the performance of a React application?
To optimize performance, you can:
- Use PureComponent or memoization to prevent unnecessary renders.
- Implement code splitting for lazy loading.
- Optimize image loading.
- Use the React DevTools for profiling.
- Minimize unnecessary re-renders using shouldComponentUpdate or React.memo.
31.What is the role of the key attribute in React lists, and why is it important?
- The key attribute is used to uniquely identify elements in a list of components. It helps React efficiently update and re-render the list when items are added, removed, or reordered.
32.Explain the concept of "lifting state up" in React
Lifting state up is a pattern where you move the state of a shared component up to a common ancestor so that multiple child components can access and modify the shared state.
33.What is Redux Thunk, and when would you use it in a Redux application?
 Redux Thunk is a middleware for Redux that allows you to write asynchronous actions in Redux. It is used when you need to perform asynchronous operations before dispatching an action, such as making API calls.
34.What is the purpose of the React Fiber architecture?
React Fiber is a reimplementation of the React core algorithm. It enables React to have better control over rendering and prioritizing updates, making it more efficient and capable of handling larger applications.
35.Explain the concept of "component lifecycle" in class components
Component lifecycle in class components refers to the different stages of a component’s existence, including mounting, updating, and unmounting. Key lifecycle methods include componentDidMount, componentDidUpdate, and componentWillUnmount.
36.What is the significance of the should Component Update method in React?
The shouldComponentUpdate method is used to control whether a component should re-render or not. It can be implemented to optimize performance by preventing unnecessary renders.
37.Explain the purpose of the create-react-app tool and how to use it
create-react-app is a tool for quickly setting up a new React application with a sensible default configuration. To use it, you can run npx create-react-app my-app and it will create a new React project with all the necessary files and dependencies.
38.What is the significance of the dangerouslySetInnerHTML attribute in React?
dangerouslySetInnerHTML is used to insert HTML into a component, but it is considered dangerous because it can expose your application to cross-site scripting (XSS) attacks. It should be used with caution and is typically avoided.
39.Explain the concept of prop types and their usage in React
Prop types are a way to specify the expected types of props passed to a component and whether they are required. They help catch runtime errors and provide documentation for component usage. You can define prop types using libraries like prop-types.
40.What is the purpose of the componentDidMount lifecycle method in React, and when is it called?
componentDidMount is called after a component is inserted into the DOM. It is often used for tasks like data fetching and interacting with the DOM. It is a good place to initialize the data that the component needs.
41.Explain the concept of PureComponent in React
PureComponent is a base class for class components in React that implements a shallow comparison of props and state to determine whether a component should re-render. It is useful for optimizing performance when you want to prevent unnecessary re-renders.
42.What are the limitations of React?
Some limitations of React include:
- It’s primarily a library for UI components and may require additional libraries or frameworks for full application development.
- Steeper learning curve for newcomers to React’s concepts and ecosystem.
- JSX can be confusing for developers new to it.
43.Explain the concept of keys in React and why they are necessary
Keys are used to give React a way to identify which elements in a list have changed, been added, or removed. They help React efficiently update the UI when rendering lists by minimizing the number of changes needed.
44.What is the significance of the useCallback hook in React, and when would you use it?
useCallback is used to memoize functions in functional components. It is beneficial when you need to optimize performance by preventing unnecessary function re-creations, especially for event handlers and props passed to child components.
45.Explain the concept of error handling in React
Error handling in React involves using error boundaries to catch and handle JavaScript errors during rendering. This prevents the entire application from crashing and allows you to display an error UI or log the error for debugging.
46.What is the purpose of the render method in class components in React?
The render method is used to define what the component should render based on its current state and props. It returns the JSX that will be converted into DOM elements.
47.Explain the concept of controlled components in React forms
Controlled components are components in which form elements (e.g., input fields, checkboxes) are controlled by React state. Changes to the form elements are handled through state, making them more predictable and easier to manage.
48.What is the significance of the componentDidUpdate lifecycle method in React, and when is it called?
componentDidUpdate is called after a component’s state or props have changed and the component has re-rendered. It is often used for side effects that depend on the updated component state or props, such as making network requests.
49.What is the purpose of the component Will Unmount lifecycle method in React, and when is it called?
component Will Unmount is called just before a component is removed from the DOM. It is often used to clean up resources, such as canceling network requests, removing event listeners, or releasing memory.
50.Explain the purpose of fragments in React and how to use them
Fragments are a way to group multiple elements in a component without adding an extra DOM element. They help keep the structure of the rendered HTML clean. You can use fragments with the <></> syntax or the <React.Fragment></React.Fragment> syntax.
51.What are React Hooks, and how have they changed the way state and side effects are handled in functional components?
React Hooks are functions that allow functional components to manage state and side effects. They have made it possible to use state, context, and side effect logic in functional components, eliminating the need for class components. Some key hooks include useState, useEffect, useContext, and useReducer.
52.Explain the differences between React's class components and functional components with Hooks
Functional components with Hooks have largely replaced class components in React. Some differences include:
- Functional components are simpler and more concise.
- Class components have lifecycle methods, while functional components use the useEffect hook.
- Class components have this binding, while functional components do not.
53.What is a higher-order component (HOC) in React, and why might you use one?
A higher-order component is a function that takes a component and returns a new component with additional props or behavior. HOCs are used for code reuse and to enhance component functionality, such as adding authentication, logging, or data fetching.
54.Explain the concept of "render props" in React and provide an example of how they can be used
Render props involve passing a function as a prop to a component, which is then called within the component to render content. This pattern allows components to share behavior and render multiple components. For example, a Mouse component can provide mouse coordinates to its children using a render prop.
55.What is server-side rendering (SSR) in React, and why might you choose to implement it in your application?
Server-side rendering is the process of rendering React components on the server and sending fully rendered HTML to the client. SSR improves SEO, initial page load performance, and provides a better user experience, especially for slow network connections.
56.Explain the concept of "reconciliation" in React's virtual DOM and how it optimizes rendering performance.
Reconciliation is the process of comparing the virtual DOM with the previous virtual DOM to determine the minimal set of changes needed to update the real DOM. React optimizes rendering by minimizing DOM manipulation and only updating what has changed.
57.What is the React Fiber architecture, and how does it impact the performance and scheduling of updates in React applications?
React Fiber is a reimplementation of the React core algorithm designed to enable better control over rendering and scheduling. It allows for asynchronous rendering, prioritizing updates, and improving overall application performance by breaking rendering work into smaller chunks.
58.Explain the purpose of the useMemo hook in React, and provide a use case where it can optimize performance
The useMemo hook is used to memoize the result of a computation and recalculate it only when its dependencies change. It is valuable for optimizing performance in scenarios where a costly calculation is based on props or state, and it doesn’t need to be recalculated on every render.
59.What is the "Context" API in React, and how does it help with state management and prop drilling?
React’s Context API is a way to share data across the component tree without passing props manually. It provides a Provider and Consumer mechanism to manage global state and avoid prop drilling, making it easier to access shared data.
60.Explain the concept of "lazy loading" in React and how it can be implemented for code-splitting and performance optimization
Lazy loading is a technique where components or modules are loaded only when they are needed, reducing the initial bundle size. It can be implemented using React’s lazy and Suspense features, making it beneficial for large applications with multiple routes or dynamic content.
61.What is the difference between controlled and uncontrolled components in React forms, and when might you choose one over the other?
Controlled components are controlled by React state, and their form data is handled through state, providing better control and validation. Uncontrolled components store form data in the DOM, which can be useful for integrating React with non-React code or working with legacy systems.
62.Explain the concept of "portals" in React and provide a use case for their usage
Portals allow rendering a component’s content at a different location in the DOM hierarchy. This is useful for scenarios where you want to render a component’s modal or tooltip outside of the component’s normal parent hierarchy to avoid CSS or stacking context issues.
63.What is React Router's "code-splitting" feature, and how does it improve the performance of your application?
React Router supports code-splitting, which allows different routes to be loaded as separate chunks. This improves application performance by reducing the initial load time, as only the required code for a specific route is fetched.
64.xplain the concept of "declarative routing" in React and how it differs from "imperative routing
Declarative routing, as used in React Router, involves defining routes and their components declaratively in a configuration file, while imperative routing typically involves manually navigating to routes using JavaScript function calls. Declarative routing is more structured and easier to maintain.
65.What is the purpose of the Profiler component in React, and how can it help you optimize your application?
The Profiler component is used for measuring and profiling the performance of a React application, helping to identify performance bottlenecks and areas for optimization. It provides insights into component rendering times and interactions.
66.Explain how React's "strict mode" works and how it can help in identifying potential issues in your application
React’s strict mode is a tool for highlighting potential problems in an application. It enables additional checks and warnings during development, including warnings about potential side effects, unnecessary renders, and deprecated features, helping to improve code quality and performance.
67.What is the significance of the ContextType in class components and the useContext hook in functional components for consuming context in React?
The ContextType and useContext are used to consume context in class and functional components, respectively. They make it easier to access context without the need for a Consumer component, improving code readability and reducing boilerplate.
68.Explain the concept of "controlled and uncontrolled forms" in React and how you can handle form validation in both scenarios
Controlled forms use React state to manage form data, making it easy to implement validation by updating state based on user input. Uncontrolled forms use refs to access form elements, and validation is typically implemented manually by checking element values.
69.What is the difference between "forward refs" and "callback refs" in React, and when might you choose one over the other?
Forward refs are created using React.forwardRef and are often used with functional components. Callback refs are created by passing a function to the ref prop and are commonly used in class components. The choice depends on the component type and your preferred syntax.
70.Explain the concept of "React Suspense" and "concurrent mode" and how they impact the performance and user experience in React applications
React Suspense is a feature that allows components to wait for something before rendering, like data loading. Concurrent mode is a set of new features that make it possible for React to work on multiple tasks simultaneously, improving performance and user experience by making applications more responsive.
71.What is the purpose of the createRoot function in React concurrent mode, and how does it impact the rendering of your application?
createRoot is used to create a concurrent root in React, allowing concurrent rendering. It separates the update and commit phases, making it possible to start rendering a tree and work on other updates concurrently. This can lead to better performance and responsiveness in applications.
72.Explain how React's "error boundaries" work and how they help prevent the entire application from crashing due to JavaScript errors
Error boundaries are components that catch JavaScript errors during rendering and prevent the entire application from crashing. They use the componentDidCatch lifecycle method to handle errors, allowing you to display an error UI or log the error for debugging.
73.What is the difference between "React.PureComponent" and "React.memo," and when might you choose one over the other for optimizing performance?
React.PureComponent is used with class components to prevent unnecessary renders by shallowly comparing state and props. React.memo is used with functional components to achieve the same result. The choice depends on whether you are using class or functional components.
74.Explain the concept of "controlled inputs" and "uncontrolled inputs" in React forms, and provide an example of each
Controlled inputs are inputs where React controls the value through state, making it easy to validate and manipulate user input. Uncontrolled inputs store their value in the DOM and are accessed through refs, which can be useful for integrating with non-React code.
75.What is the purpose of the "useLayoutEffect" hook in React, and how does it differ from the "useEffect" hook?
useLayoutEffect is similar to useEffect but is synchronous and fires before the browser’s paint phase. It is useful for tasks that require immediate updates to the DOM layout, such as measuring elements or managing animations.
76.Explain the concept of "Refs and the DOM" in React and provide a use case where you might need to use refs to access a DOM element
Refs provide a way to access and manipulate a DOM element directly in React. They can be used for tasks like focusing an input field, scrolling to a specific element, or integrating third-party libraries that work directly with the DOM.
77.What is the purpose of the "useDebugValue" hook in React, and how can it be helpful during development?
use Debug Value is a custom hook that allows you to provide additional debug information for custom hooks. It is helpful for displaying information about a custom hook in development tools, making it easier to understand and debug hooks.
78.Explain the concept of "controlled components" and "uncontrolled components" in React forms, and provide an example of each
 Controlled components are form elements in which the value is controlled by React state. Changes to the element value trigger state updates, allowing for validation and controlled behavior. Uncontrolled components store their value in the DOM, and you access their values through refs, which can be useful for integrating with non-React code.
79.What is the "Accessibility Tree" in React, and how can you ensure your application is accessible to all users?
The Accessibility Tree is a representation of the UI in terms of accessibility. To ensure accessibility, you can use semantic HTML elements, provide meaningful text alternatives for images and icons, manage focus and keyboard navigation, and test with screen readers and accessibility tools.
80.Explain the purpose of "React Fragments" and provide a use case for when you might want to use them in your components
React Fragments allow you to group multiple elements without adding extra DOM nodes. They are useful when you want to return adjacent elements from a component without wrapping them in a parent element, like in a table row or a list.
81.What are "React Suspense" and "concurrent mode," and how do they improve the performance and user experience of React applications?
React Suspense is a feature that allows components to wait for something, such as data loading, before rendering. Concurrent mode is a set of features that enable React to work on multiple tasks simultaneously, improving application performance and responsiveness by making applications more concurrent and efficient.
82.What is the "Cache API" in React, and how can it be used to manage data caching for improved application performance?
The Cache API is a web API used to store and retrieve responses in a cache. It can be used in React applications to implement data caching strategies, such as cache-first or stale-while-revalidate, for better performance and reduced network requests.
83.Explain the purpose of "Immutable Data Structures" in React and how they can help optimize application performance
Immutable data structures are data collections that cannot be modified after creation. They help prevent unexpected side effects and make it easier to track changes in a React application, improving performance and predictability.
84.What are "React Suspense for Data Fetching" and "React Query," and how can they simplify data fetching and state management in React applications?
React Suspense for Data Fetching is a pattern in React that allows components to wait for data before rendering, while React Query is a library that simplifies data fetching, caching, and state management in React applications. Together, they streamline data-related tasks, improving application performance and reducing complexity.
85.What are the benefits of using "Hooks" for state management and side effects in React, compared to traditional class components?
 Hooks provide a more concise and predictable way of managing state and side effects in functional components, reducing the need for class components. They simplify code, improve reusability, and are easier to test and maintain.
86.Explain the concept of "React Portals" and provide a use case where you might use them in your application
React Portals allow you to render components outside the parent hierarchy, making it useful for rendering modals, tooltips, or dropdown menus that need to appear over other elements on the page without causing CSS or stacking issues.
87.What is the "useContext" hook in React, and how can it be used for state management in a component tree?
The useContext hook allows components to access a context’s value, eliminating the need to use a Consumer component for context data. It simplifies state management in complex component trees and makes it easier to access shared data.
86.What is React.memo in React and how can it help in optimizing functional components?
React.memo is a higher-order component in React used to memoize functional components, preventing unnecessary re-renders. It compares the previous props with the new props and only re-renders the component if there are differences. This can be valuable for optimizing performance in functional components.
87.Explain the concept of "Render Props" and how they enable component composition and code reusability in React. Provide an example of using a Render Prop
Render Props is a design pattern in React where a component’s behavior is provided as a function via a prop. It allows component composition and code reusability. An example could be a Toggle component that takes a render prop function, enabling the user to customize the rendering of the component based on the state.
90.What is the Virtual DOM, and how does it contribute to the efficiency of React's rendering process?
The Virtual DOM is a lightweight representation of the actual DOM. React uses it to efficiently update the UI by comparing changes in the Virtual DOM and applying minimal updates to the real DOM. This minimizes costly DOM manipulation and enhances rendering efficiency.
91.Explain how React Router's nested routing works, and provide an example of a use case where you would need to use it
Nested routing in React Router involves defining child routes within the parent route. It allows components to be nested and can be used for scenarios like creating sub-routes within a dashboard or user profile page, each with its own set of routes.
92.What is server-side rendering (SSR) in React, and how does it differ from client-side rendering (CSR)? When might you choose SSR over CSR for your application?
Server-side rendering (SSR) in React involves rendering components on the server and sending fully rendered HTML to the client, whereas client-side rendering (CSR) renders components on the client-side using JavaScript. SSR is chosen for improved SEO, initial page load performance, and situations where rendering on the server is more efficient.
93.Explain the concept of "reconciliation" in React's Virtual DOM and how it optimizes the rendering process. What are the key algorithms involved in reconciliation?
Reconciliation in React involves comparing the new Virtual DOM with the previous one to determine the minimal set of changes needed to update the real DOM. It uses algorithms like tree differencing and heuristics to efficiently identify changes, minimizing the amount of work required to update the UI.
94.What is the Context API in React, and how does it compare to Redux for state management? When might you choose one over the other?
The Context API in React allows components to share data without manually passing props. Redux is a state management library. The choice between them depends on the application’s complexity. The Context API is suitable for smaller applications, while Redux is preferred for larger, more complex apps with extensive state management needs.
95.Explain the concept of "React Fiber" and how it improves the efficiency of React's rendering process
React Fiber is a reimplementation of React’s core algorithm. It enhances the rendering process by enabling asynchronous rendering, better scheduling of updates, and breaking rendering work into smaller units. This improves the application’s performance and responsiveness.
96.What is the "memoization" technique, and how can it be used to optimize the performance of React components? Provide an example of memoization in a React application
Memoization is a technique used to cache the results of expensive computations, making it faster to access them in subsequent calls. In React, you can use memoization to optimize expensive calculations within components, such as complex rendering logic, by storing the results and returning them from cache when needed.
97.Explain the concept of "suspense" in React and how it enables better handling of asynchronous data and code-splitting
Suspense in React allows components to pause rendering until some condition is met, such as data loading or code splitting. This makes it possible to create more predictable and responsive user interfaces, as it can coordinate the timing of when data becomes available for rendering.
98.What is the "Strict Mode" in React, and how can it help identify potential issues in your application during development?
React’s Strict Mode is a development tool that highlights potential problems and warns about deprecated features in the application. It provides additional checks and warnings during development, helping to improve code quality, performance, and development experience.
99.Explain the concept of "Server Components" in React and how they enable server-side rendering for parts of a page. What are the potential benefits of using Server Components?
Server Components in React allow parts of a page to be rendered on the server while other parts are rendered on the client. This approach can improve performance, as only the necessary parts of the page are re-rendered when changes occur. It also enables dynamic, interactive updates to server-rendered content.
100.Explain the concept of "controlled and uncontrolled components" in React forms, and provide a real-world scenario where you might choose one approach over the other
Controlled components are React components where the form data is controlled by React state, and user input triggers state updates. In contrast, uncontrolled components store form data in the DOM, and you access it using refs.
A real-world scenario for using controlled components is when you have a form that requires validation, dynamic interactions, or immediate feedback based on user input. Controlled components allow you to maintain full control over the form’s behavior and provide a centralized place to manage the form’s state and validation. Uncontrolled components might be used when integrating React with non-React code or when dealing with large forms with many input fields, as they can reduce the amount of code needed for state management.