If you’re gearing up for a ReactJS interview in 2025, you’ve come to the right place. ReactJS is one of the most popular libraries for building amazing web apps, and knowing it well can open doors to some fantastic opportunities. In this blog, we\u2019ve compiled 100 of the most common ReactJS interview questions you might face, ranging from beginner-level basics to advanced topics. Whether you\u2019re just starting or a pro brushing up for a tough interview, this guide has covered you. Let\u2019s start.<\/p>\n\n\n\n
ReactJS<\/a> is a popular JavaScript library developed by Facebook (now Meta) for building user interfaces, especially for web applications. It focuses on making the creation of dynamic, fast, and responsive UI components easier.<\/p>\n\n\n\n React\u2019s big selling point? It lets you build web apps where users can interact with different parts of the page without reloading the whole thing. Think of it like giving your website a superpower to feel fast and smooth.<\/p>\n\n\n\n These questions are perfect for beginners getting started with ReactJS<\/a>.<\/p>\n\n\n\n A:<\/strong> JSX is a syntax extension for JavaScript. It lets you write HTML-like code inside your JavaScript files, making it easier to design UI components.<\/p>\n\n\n\n A:<\/strong> A React component is like a building block of your app. It\u2019s a small, reusable piece of code that controls how a part of your UI looks and works.<\/p>\n\n\n\n A:<\/strong> Functional components are simple JavaScript functions that return JSX. Class components are ES6 classes with more features, but functional components are preferred nowadays because of React Hooks.<\/p>\n\n\n\n A:<\/strong> Props (short for properties) are like inputs you pass to a component. They help you send data from one component to another.<\/p>\n\n\n\n A:<\/strong> State is an object in a component that holds data that can change over time. For example, a counter value or user input can be stored in the state.<\/p>\n\n\n\n A:<\/strong> Props:<\/strong> Passed to a component and are read-only.<\/p>\n\n\n\n State:<\/strong> Managed inside a component and can be updated.<\/p>\n\n\n\n A:<\/strong> The render() method tells React what to display on the screen. It returns the JSX for your component.<\/p>\n\n\n\n A:<\/strong> A React fragment (<React.Fragment> or <>) lets you group multiple elements without adding extra HTML elements like <div>.<\/p>\n\n\n\n A:<\/strong> The virtual DOM is a lightweight copy of the real DOM. React uses it to calculate changes and only updates what\u2019s necessary, making updates faster.<\/p>\n\n\n\n A:<\/strong> You pass data using props. For example: A:<\/strong> A key helps React identify which list items have changed, making updates efficient.<\/p>\n\n\n\n A:<\/strong> useState is a React Hook that lets you add state to a functional component. Example: A:<\/strong> Events in React are like browser events (e.g., onClick, onChange). You can attach them to elements to handle user actions.<\/p>\n\n\n\n A:<\/strong> You can use state to manage form inputs and handle the onSubmit event to process form data.<\/p>\n\n\n\n A:<\/strong> defaultProps lets you set default values for props if they aren\u2019t provided. Example: A:<\/strong> You can use JavaScript conditions like if or the ternary operator. Example: A:<\/strong> It\u2019s a React method that creates a virtual DOM element. JSX is just a shorthand for this.<\/p>\n\n\n\n A:<\/strong> Controlled Components:<\/strong> Manage their value through state (e.g., <input> with value controlled by state).<\/p>\n\n\n\n Uncontrolled Components:<\/strong> Use refs to access their value directly from the DOM.<\/p>\n\n\n\n A:<\/strong> Use the setState method in class components or the setState function from useState in functional components.<\/p>\n\n\n\n A:<\/strong> React makes it easier to build fast, interactive user interfaces with reusable components and efficient updates using the virtual DOM.<\/p>\n\n\n\n These questions should help beginners get a good grasp of React\u2019s basics. Let\u2019s move on to intermediate concepts!<\/p>\n\n\n\n Here\u2019s a collection of questions that dive deeper into React concepts like lifecycle methods, hooks, and conditional rendering.<\/p>\n\n\n\n A:<\/strong> The virtual DOM is a lightweight version of the real DOM. React updates it first, calculates the differences (or \u201cdiffing\u201d), and only updates the parts of the real DOM that changed.<\/p>\n\n\n\n A:<\/strong> Lifecycle methods are functions you can override in class components to run code at specific stages of a component\u2019s life, like mounting, updating, or unmounting.<\/p>\n\n\n\n A:<\/strong> componentDidMount: Runs after the component is added to the DOM.<\/p>\n\n\n\n componentDidUpdate: Runs after the component updates.<\/p>\n\n\n\n componentWillUnmount: Runs before the component is removed from the DOM.<\/p>\n\n\n\n A:<\/strong> React Hooks like useEffect can handle side effects, making lifecycle methods less necessary in functional components.<\/p>\n\n\n\n A:<\/strong> useEffect: Runs after the DOM updates. It\u2019s great for fetching data or subscriptions.<\/p>\n\n\n\n useLayoutEffect: Runs before the DOM updates, useful for layout adjustments.<\/p>\n\n\n\n A:<\/strong> Return a cleanup function from useEffect. Example: const interval = setInterval(() => console.log(“tick”), 1000);<\/p>\n\n\n\n return () => clearInterval(interval); \/\/ Cleanup<\/p>\n\n\n\n }, []);<\/p>\n\n\n\n A:<\/strong> It\u2019s displaying different content based on a condition. Use JavaScript logic like if or the ternary operator in your JSX.<\/p>\n\n\n\n A: A:<\/strong> useReducer is a Hook for managing complex state logic, similar to Redux. It takes a reducer function and an initial state.<\/p>\n\n\n\n A:<\/strong> Use useMemo to memoize a value so it doesn\u2019t get recalculated unless its dependencies change. It helps optimize performance. A:<\/strong> useCallback memoizes a function so it doesn\u2019t get recreated unnecessarily, which is useful for preventing unnecessary renders in child components.<\/p>\n\n\n\n A:<\/strong> The key prop is used in lists to help React identify which items have changed, added, or removed. It ensures efficient updates.<\/p>\n\n\n\n A:<\/strong> The cleanup function in Example:<\/strong><\/p>\n\n\n\n A:<\/strong> React uses controlled components for forms, where the input\u2019s value is tied to state. <input value={name} onChange={(e) => setName(e.target.value)} \/>;<\/p>\n\n\n\n A:<\/strong> The Context API allows you to share data (like theme or user info) across components without passing props down manually.<\/p>\n\n\n\n A: <\/strong>Create a context: const MyContext = React.createContext();<\/p>\n\n\n\n Provide the context: <MyContext.Provider value={data}><\/p>\n\n\n\n Consume the context: useContext(MyContext)<\/p>\n\n\n\n A:<\/strong> HOCs are functions that take a component and return a new component with additional functionality. They\u2019re used for reusing logic. return function (props) {<\/p>\n\n\n\n return isLoggedIn ? <WrappedComponent {…props} \/> : <Login \/>;<\/p>\n\n\n\n };<\/p>\n\n\n\n }<\/p>\n\n\n\n A:<\/strong> It helps highlight potential problems in your React app during development, like unsafe lifecycle methods or deprecated features.<\/p>\n\n\n\n A:<\/strong> Use React.memo to prevent unnecessary re-renders.<\/p>\n\n\n\n Use useMemo and useCallback for expensive calculations and functions.<\/p>\n\n\n\n Avoid inline functions and objects in JSX.<\/p>\n\n\n\n A:<\/strong> It schedules a state update. React re-renders the component with the new state value. However, state updates are asynchronous, so you won\u2019t see the new state immediately.<\/p>\n\n\n\n These intermediate questions give you a good handle on React\u2019s more complex concepts.<\/p>\n\n\n\n Let\u2019s explore some advanced React concepts like design patterns, architecture, and optimization techniques.<\/p>\n\n\n\n A:<\/strong> Code splitting is a technique to break your code into smaller bundles. This allows React to load only the necessary parts of your app when needed, improving performance. You can implement it using React’s lazy and Suspense.<\/p>\n\n\n\n A:<\/strong> Use the React.lazy function to load components on demand, combined with Suspense for fallback loading UI. <Suspense fallback={<div>Loading…<\/div>}><\/p>\n\n\n\n <LazyComponent \/><\/p>\n\n\n\n <\/Suspense><\/p>\n\n\n\n A:<\/strong> SSR generates the HTML for your app on the server instead of the browser. This improves SEO and makes your app load faster for users. Frameworks like Next.js make SSR easy.<\/p>\n\n\n\n A:<\/strong> Portals let you render children into a DOM node outside the parent component’s hierarchy. It\u2019s great for modals and tooltips. A:<\/strong> Context API<\/strong>: Simple state sharing for smaller apps.<\/p>\n\n\n\n<\/a>Key Features of ReactJS<\/strong><\/h3>\n\n\n\n
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<\/a>Use Cases of ReactJS in 2025<\/strong><\/h3>\n\n\n\n
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Basic ReactJS Questions and Answers<\/strong><\/h2>\n\n\n\n
Q: What is JSX?<\/strong><\/h4>\n\n\n\n
Q: What is a React component?<\/strong><\/h4>\n\n\n\n
Q: What\u2019s the difference between functional and class components?<\/strong><\/h4>\n\n\n\n
Q: What are props in React?<\/strong><\/h4>\n\n\n\n
Q: What is state in React?<\/strong><\/h4>\n\n\n\n
Q: How is state different from props?<\/strong><\/h4>\n\n\n\n
Q: What is the purpose of render() in a React class component?<\/strong><\/h4>\n\n\n\n
Q: What is a React fragment?<\/strong><\/h4>\n\n\n\n
Q: Can you explain the virtual DOM?<\/strong><\/h4>\n\n\n\n
Q: How do you pass data from a parent to a child component?<\/strong><\/h4>\n\n\n\n
<ChildComponent name=”John” \/><\/p>\n\n\n\nQ: What is the use of key in lists?<\/strong><\/h4>\n\n\n\n
Q: What does useState do in React?<\/strong><\/h4>\n\n\n\n
const [count, setCount] = useState(0)<\/p>\n\n\n\nQ: What are events in React?<\/strong><\/h4>\n\n\n\n
Q: How do you handle forms in React?<\/strong><\/h4>\n\n\n\n
Q: What is defaultProps in React?<\/strong><\/h4>\n\n\n\n
MyComponent.defaultProps = { color: “blue” };<\/p>\n\n\n\nQ: How do you conditionally render content in React?<\/strong><\/h4>\n\n\n\n
{isLoggedIn ? <Dashboard \/> : <Login \/>}<\/p>\n\n\n\nQ: What does React.createElement do?<\/strong><\/h4>\n\n\n\n
Q: What are controlled and uncontrolled components?<\/strong><\/h4>\n\n\n\n
Q: How do you update the state in React?<\/strong><\/h4>\n\n\n\n
Q: Why do we need React?<\/strong><\/h4>\n\n\n\n
<\/a>Intermediate ReactJS Questions and Answers<\/strong><\/h2>\n\n\n\n
Q: How does the virtual DOM work in React?<\/strong><\/h4>\n\n\n\n
Q: What are lifecycle methods in React?<\/strong><\/h4>\n\n\n\n
Q: Can you name some common lifecycle methods?<\/strong><\/h4>\n\n\n\n
Q: What replaced lifecycle methods in functional components?<\/strong><\/h4>\n\n\n\n
Q: What is the difference between useEffect and useLayoutEffect?<\/strong><\/h4>\n\n\n\n
Q: How do you clean up effects in useEffect?<\/strong><\/h4>\n\n\n\n
useEffect(() => {<\/p>\n\n\n\nQ: What is conditional rendering in React?<\/strong><\/h4>\n\n\n\n
Q: How do you write conditional rendering using a ternary operator?<\/strong><\/h4>\n\n\n\n
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{isLoggedIn ? <Dashboard \/> : <Login \/>}<\/p>\n\n\n\nQ: What does useReducer do in React?<\/strong><\/h4>\n\n\n\n
Q: When would you use useMemo?<\/strong><\/h4>\n\n\n\n
Example:
const computedValue = useMemo(() => expensiveCalculation(a, b), [a, b]);<\/p>\n\n\n\nQ: What is useCallback used for?<\/strong><\/h4>\n\n\n\n
Q: Can you explain the key prop in React?<\/strong><\/h4>\n\n\n\n
Q: What is the significance of the
useEffect<\/code> cleanup function in React?<\/strong><\/h4>\n\n\n\nuseEffect<\/code> is essential for preventing memory leaks and unwanted side effects in React components. When useEffect<\/code> returns a function, React runs that function before executing the next effect and when the component unmounts. This is particularly useful for tasks like:<\/p>\n\n\n\n\n
useEffect(() => {\n const timer = setInterval(() => {\n console.log(\"Interval running...\");\n }, 1000);\n\n return () => {\n clearInterval(timer); \/\/ Cleanup function\n console.log(\"Interval cleared\");\n };\n}, []);<\/code><\/pre>\n\n\n\nQ: How does React handle forms?<\/strong><\/h4>\n\n\n\n
Example:
const [name, setName] = useState(“”);<\/p>\n\n\n\nQ: What is the Context API in React?<\/strong><\/h4>\n\n\n\n
Q: How do you use the Context API?<\/strong><\/h4>\n\n\n\n
Q: What are Higher-Order Components (HOCs)?<\/strong><\/h4>\n\n\n\n
Example:
function withAuth(WrappedComponent) {<\/p>\n\n\n\nQ: What is the purpose of React.StrictMode?<\/strong><\/h4>\n\n\n\n
Q: How do you optimize rendering in React?<\/strong><\/h4>\n\n\n\n
Q: What happens if you call setState in React?<\/strong><\/h4>\n\n\n\n
<\/a>Advanced ReactJS Questions and Answers<\/strong><\/h2>\n\n\n\n
Q: What is code splitting in React?<\/strong><\/h4>\n\n\n\n
Q: How do you implement lazy loading in React?<\/strong><\/h4>\n\n\n\n
Example:
const LazyComponent = React.lazy(() => import(‘.\/MyComponent’));<\/p>\n\n\n\nQ: What is server-side rendering (SSR) in React?<\/strong><\/h4>\n\n\n\n
Q: What are portals in React?<\/strong><\/h4>\n\n\n\n
Example:
ReactDOM.createPortal(child, document.getElementById(‘portal-root’));<\/p>\n\n\n\nQ: What is the difference between context and Redux?<\/strong><\/h4>\n\n\n\n