Here are the top 30 most asked React interview questions along with proper explanations:
What is React and how does it work?
React is a JavaScript library for building user interfaces. It allows developers to create reusable UI components and efficiently update the user interface when the underlying data changes. React uses a virtual DOM (Document Object Model) to optimize rendering performance by minimizing direct manipulation of the actual DOM.
What are the key features of React?
Some key features of React include:
Virtual DOM: React uses a virtual DOM to update and render components efficiently.
Component-based architecture: React follows a component-based approach, where UIs are divided into reusable and independent components.
One-way data flow: React enforces one-way data flow, which means data changes from parent components to child components.
JSX: React uses JSX (JavaScript XML) syntax, which allows mixing HTML-like syntax within JavaScript code.
What are the differences between functional components and class components in React?
Functional components are defined as JavaScript functions, while class components are defined as JavaScript classes that extend the
React.Componentclass. Functional components are simpler and easier to test, while class components offer more features like lifecycle methods and local component state.
What are React hooks?
React hooks are functions that allow functional components to have state and lifecycle features. Some commonly used hooks are:
useState: Manages state in functional components.useEffect: Performs side effects (such as API calls) in functional components.useContext: Accesses the nearest Context object.useRef: Creates a mutable reference that persists across renders.
What is the purpose of keys in React lists?
Keys are used in React lists to help identify which items have changed, been added, or been removed. They provide a unique identifier to each list item, enabling React to optimize the rendering process by only updating the necessary items instead of re-rendering the entire list.
What is the role of the setState method in React?
The setState method is used to update the state of a React component. It allows you to modify the component's state and triggers a re-rendering of the component with the updated state. React may batch multiple setState calls into a single update for performance reasons.
What is JSX in React?
JSX is a syntax extension for JavaScript that allows you to write HTML-like code within JavaScript. It is used in React to define the structure and content of components. JSX is not required to use React, but it provides a more readable and declarative way to define UI components.
What is the significance of the componentDidMount lifecycle method in React?
The componentDidMount lifecycle method is called once, immediately after a component is mounted (inserted into the DOM). It is commonly used to perform initialization tasks, such as fetching data from an API, subscribing to events, or setting up timers.
How can you optimize the performance of React applications?
Some techniques to optimize React application performance include:
Using React's virtual DOM efficiently.
Implementing shouldComponentUpdate or React.memo to prevent unnecessary re-renders.
Code splitting and lazy loading to load components only when needed.
Using production builds and minimizing the bundle size.
Implementing memoization or caching for expensive operations.
How does React differ from Angular or Vue.js?
React, Angular, and Vue.js are all popular JavaScript frameworks for building web applications. Here are some key differences:
React is a library, while Angular and Vue.js are frameworks.
React uses a virtual DOM, while Angular and Vue.js use a real DOM. - React focuses on UI components and encourages component reuse, while Angular and Vue.js provide more comprehensive solutions for building applications. - React relies on JavaScript, while Angular uses TypeScript, and Vue.js supports both JavaScript and TypeScript.
What is the purpose of React Router, and how does it work?
React Router is a popular library for handling routing in React applications. It allows developers to create single-page applications with multiple views and navigational features. React Router uses declarative routing, where routes are defined as components and rendered based on the current URL.
What is the significance of the shouldComponentUpdate method in React?
The shouldComponentUpdate method is a lifecycle method that allows developers to optimize the rendering process by determining if a component should update or not. By implementing this method, you can control when a component should re-render based on changes in props or state.
What is the purpose of Redux in React applications?
Redux is a predictable state container for JavaScript applications, including React. It provides a centralized state management solution, making it easier to manage and share data across different components. Redux follows a unidirectional data flow and encourages immutability.
What are React controlled components and uncontrolled components?
In React, controlled components are components whose form elements (like inputs) are controlled by React state. The component's state manages the value and handles changes to the form elements. In contrast, uncontrolled components store their own state internally and manage the form elements themselves.
What is the concept of lifting state up in React?
Lifting state up is a pattern in React where multiple components share and manage the same state by moving it to a common ancestor component. This allows sibling components to communicate and synchronize their state by passing props and callbacks through the parent component.
Explain the concept of conditional rendering in React.
Conditional rendering in React refers to the ability to render different components or content based on certain conditions. This is achieved by using JavaScript expressions or conditional statements within the JSX code to determine which component or content to render.
What are React fragments, and why are they useful?
React fragments are a way to group multiple elements together without adding an extra DOM element. They provide a lightweight way to group elements and avoid unnecessary HTML markup. Fragments are especially useful when returning multiple elements from a component's render method.
What is the purpose of the useEffect hook in React?
The useEffect hook is used to handle side effects in functional components. It allows you to perform actions such as data fetching, subscriptions, or DOM manipulations after the component has rendered. The useEffect hook can also clean up these side effects when the component unmounts.
How can you pass data between components in React?
Data can be passed between components in React through props. Props are passed from parent components to child components, allowing data to flow in a unidirectional manner. Alternatively, React's Context API can be used to share data between components without passing props through every intermediate component.
What is the significance of the key prop in React lists, and why should it be unique?
The key prop is used in React lists to provide a unique identifier for each item in the list. React uses the key prop to efficiently update and re-render the list by identifying which items have changed, been added, or been removed. The key prop should be unique among sibling elements to ensure proper reconciliation.
What are React refs, and how are they used?
React refs are a way to access and interact with DOM elements or React components directly. They provide a way to obtain references to specific elements or components and manipulate them imperatively. Refs are commonly used for focusing input fields, triggering animations, or integrating with third-party libraries.
What is the significance of the context in React, and when should it be used?
Context in React is an advanced feature that allows data to be passed through the component tree without explicitly passing props at every level. It is used to share data that is considered "global" or shared among multiple components, such as user authentication, theme settings, or localization.
What is React's reconciliation process?
React's reconciliation process is the mechanism by which React determines the changes that need to be made to the DOM in response to changes in component props or state. React uses a diffing algorithm to compare the previous and current virtual DOM representations and efficiently update only the necessary parts of the DOM.
What is the purpose of the forwardRef function in React?
The forwardRef function in React is used to forward a ref from a child component to a DOM element or another component. It enables parent components to directly access and interact with a child component's underlying DOM element or component instance.
What are React hooks rules or guidelines to follow?
Some important rules and guidelines for using React hooks effectively are:
Hooks should only be called at the top level of a functional component or inside other custom hooks.
Hooks should not be called conditionally.
Hooks should always be called in the same order in every render.
Avoid using hooks inside loops or nested functions.
What is the purpose of the memo function in React, and how does it optimize component rendering?
The memo function is a higher-order component (HOC) in React that helps optimize functional components by preventing unnecessary re-renders. It memoizes the component's result and re-renders only if the component's props have changed. It is especially useful for optimizing performance when working with large lists or complex components.
What is the significance of error boundaries in React, and how are they implemented?
Error boundaries in React are components that catch JavaScript errors during rendering, lifecycle methods, and in the constructors of the whole component tree below them. They help prevent the entire React application from crashing due to unhandled errors. Error boundaries are implemented using the componentDidCatch lifecycle method.
What are the differences between controlled and uncontrolled components in forms?
Controlled components are form elements (like inputs or selects) whose values are controlled by React state. Their values are updated and managed through the component's state, and changes to the value are handled by React event handlers. Uncontrolled components, on the other hand, manage their own state internally, and their values are accessed through references after form submission.
What is the purpose of the useReducer hook in React, and how is it used?
The useReducer hook is used to manage complex state logic in functional components. It is an alternative to the useState hook when the state logic involves multiple sub-values or requires more advanced update rules. useReducer follows the same principles as Redux, with a state and an action that specifies how to update the state.
How can you handle forms in React?
Forms in React can be handled by utilizing controlled components, where the form element values are controlled by React state. The form input values are updated through event handlers, and form submission is handled by preventing the default form submission