React Interview Question And Answer :-
React question and answer
Explain the virtual DOM in React and how it improves performance.
Answer: The virtual DOM is a lightweight copy of the actual DOM maintained by React. When changes are made to the state of a component, React first updates the virtual DOM instead of the actual DOM. Then, it performs a “diffing” process to identify the changes between the previous virtual DOM and the updated virtual DOM. Finally, it updates only the necessary parts of the actual DOM, reducing the number of DOM manipulations and improving performance.
What are the differences between functional components and class components in React? When would you use one over the other?
Answer: Functional components are simpler and more concise, written as pure functions, while class components are ES6 classes extending from React.Component. Functional components are preferred for their simplicity, reusability, and easier testing. Class components are used when state or lifecycle methods are needed.
How does React handle state management? Can you explain the difference between props and state?
Answer:React manages state using the useState hook or by extending React.Component class in class components. Props are immutable data passed from parent to child components, while state represents mutable data managed within a component.
What is JSX, and why is it used in React?
Answer:
Answer:JSX is a syntax extension for JavaScript that allows mixing HTML-like syntax within JavaScript code. It simplifies the creation of React elements and improves readability by resembling HTML, making it easier to visualize the UI structure.
How do you handle events in React? Provide examples of commonly used events.
Answer:Events in React are handled using camelCase event names such as onClick or onChange events.
Example:
What is the purpose of keys in React lists?
Answer:Keys are used to uniquely identify elements in a list. They help React identify which items have changed, are added, or are removed, improving the performance and efficiency of list rendering.
How do you optimize performance in React applications?
Answer:Performance optimization techniques include using PureComponent or memoization for components, minimizing re-renders, implementing lazy loading, code splitting, and using libraries like React.memo or useMemo.
What is Redux, and how does it differ from React’s built-in state management?
Answer:Redux is a predictable state container for JavaScript applications, commonly used with React for managing application state in large or complex applications. It differs from React’s built-in state management by providing a global state accessible across components and enabling predictable state mutations through reducers.
What are hooks in React, and how do they differ from class-based components? Provide examples of some commonly used hooks.
Answer:Hooks are functions that enable using state and other React features in functional components. They provide a more flexible and concise way to use state and lifecycle features compared to class components. Examples include useState, useEffect, and useContext.
What are higher-order components (HOCs) and how do they work?
Answer: Higher-order components are functions that take a component and return a new component with enhanced functionality. They allow code reuse, logic abstraction, and composability in React applications. HOCs work by accepting a component as an argument, creating a new component that wraps the original component, and passing additional props or behavior to the wrapped component.
Compare and contrast controlled vs. uncontrolled components in React forms.
Answer: Controlled components are React components where form data is controlled by React state. The value of the form elements (e.g., input, textarea) is controlled by React, making it easy to manipulate and validate form data. Uncontrolled components, on the other hand, allow form data to be handled by the DOM itself, using refs to access form elements. Controlled components provide better control and validation, while uncontrolled components may be faster for simple forms.
What is React context and when would you use it?
Answer: React context provides a way to pass data through the component tree without having to pass props manually at every level. It’s useful for sharing data that needs to be accessible by many components at different nesting levels. Context is often used for themes, localization, and user authentication data.
Discuss the differences between React’s class components and functional components with hooks.
Answer: Class components are ES6 classes that extend React.Component and manage state using this.state. Functional components are simpler, function-based components that don’t have their own state until the introduction of hooks. With the introduction of hooks, functional components can now manage state and perform side effects using useState, useEffect, and other hooks, making them more concise and easier to understand.
Explain the concept of reconciliation in React.
Answer: Reconciliation is the process by which React updates the DOM to match the virtual DOM after a component’s state or props change. React uses a diffing algorithm to compare the previous virtual DOM with the new virtual DOM and only applies the necessary changes to the actual DOM, minimizing unnecessary DOM manipulations for improved performance.
What are React hooks, and why were they introduced? Provide examples of some commonly used hooks.
Answer: React hooks are functions that enable functional components to use state and lifecycle features previously only available in class components. They were introduced to simplify the creation and management of stateful logic in functional components. Examples of commonly used hooks include useState for managing state, useEffect for handling side effects, useContext for accessing context, and useMemo for memoizing expensive calculations.
How does React Router work, and how would you implement dynamic routing?
Answer: React Router is a library that enables declarative routing in React applications. It provides components like BrowserRouter and Route to define routes and render corresponding components based on the URL. Dynamic routing involves defining routes dynamically based on certain conditions, such as user authentication status or data availability. This can be achieved by conditionally rendering Route components or using higher-order components to guard routes.
What is server-side rendering (SSR) in React, and what are its benefits?
Answer: Server-side rendering is the process of rendering React components on the server and sending the generated HTML to the client, instead of relying on client-side JavaScript to render components. SSR improves performance by reducing the time to first paint and time to interactive, enhances SEO by serving pre-rendered HTML to search engines, and provides better support for clients with limited JavaScript capabilities.
Discuss the role of Redux in React applications and when it’s appropriate to use it.
Answer: Redux is a predictable state container for JavaScript applications, commonly used with React to manage application state in a centralized manner. It’s appropriate to use Redux in large-scale applications with complex state management needs, such as applications with deeply nested component trees, multiple sources of truth, or asynchronous data fetching requirements.
Explain the concept of lazy loading in React and how you would implement it.
Answer: Lazy loading is the technique of deferring the loading of non-essential resources (such as components or data) until they are actually needed. In React, lazy loading is typically implemented using dynamic imports and the React.lazy function. Components that should be lazily loaded are wrapped in React.lazy, which dynamically imports the component when it’s rendered for the first time, splitting the bundle and reducing initial loading time.
What are key differences between React’s useEffect and componentDidMount/componentDidUpdate lifecycle methods?
Answer: useEffect is a hook used in functional components to perform side effects, such as data fetching, component updates, or subscriptions. It replaces componentDidMount, componentDidUpdate, and componentWillUnmount lifecycle methods from class components. The key differences include the ability to perform multiple side effects in a single useEffect, the ability to control when the effect runs using dependency arrays, and the absence of a separate cleanup phase (handled within the useEffect function).
How would you optimize a React application for performance?
Answer: Performance optimization in React involves various techniques such as code splitting, lazy loading, memoization, minimizing re-renders using PureComponent or React.memo, using the shouldComponentUpdate lifecycle method or the useMemo hook, optimizing network requests, and implementing server-side rendering (SSR) or static site generation (SSG) for initial page loads.
Explain the concept of synthetic events in React and why they are used.
Answer: Synthetic events are cross-browser wrappers around the browser’s native events provided by React. They offer a consistent interface for handling events across different browsers and platforms. Synthetic events provide additional features like event pooling for performance optimization, automatic event delegation, and normalization of event properties to ensure consistent behavior.
What are some common patterns for managing state in React applications, and when would you use each?
Answer: Common state management patterns in React include lifting state up, where state is managed in a common ancestor component and passed down as props to child components; using context to share state between components without having to pass props through intermediate components; and using libraries like Redux or MobX for centralized state management in large-scale applications with complex state requirements. The choice of pattern depends on the size, complexity, and specific requirements of the application.
