Top 40+ React JS Interview Questions with Detailed Answers for Freshers

Are you preparing for React Js interview? To help you out, we complied list of over 40+ crucial React JS interview questions with detailed answers for freshers. This guide aims to equip you with the knowledge needed to confidently tackle any interview scenario.

React JS Interview Questions and Answers for Freshers

1. What is React JS?
2. What are the key features of React JS?
3. What is JSX? Why do we use it?
4. What is the Virtual DOM? How does it work?
5. Explain the difference between Real DOM and Virtual DOM.
6. How do you create components in React?
7. What are props in React?
8. What are state and lifecycle methods in React?
9. What is Redux and how does it relate to React?
10. Explain how you can handle forms in React.
11. What are hooks in React?
12. What is prop drilling? How can it be avoided?
13. How does error handling work in React?
14. What are controlled vs uncontrolled components?
15. What is routing in React? Why do we need it?
16. How do you optimize performance in a React application?
17. What is Context API? When would you use it?
18. Explain Higher Order Components (HOCs).
19. What are refs in React? When would you use them?
20. What are synthetic events in React?
21. What is the purpose of keys in React?
22. What is a React fragment, and why would you use it?
23. How do you handle events in React?
24. Explain React’s reconciliation process.
25. What is conditional rendering in React? How do you implement it?
26. What is the purpose of the useEffect hook?
27. How can you make an HTTP request in a React app?
28. What is the purpose of the useState hook?
29. What are controlled and uncontrolled inputs in React?
30. Explain the concept of memoization in React.
31. How does React handle updates in child components?
32. What is lazy loading in React, and how is it implemented?
33. How do you pass data between sibling components in React?
34. What is code-splitting in React?
35. How do you handle errors in a React component?
36. What are default props in React?
37. What is the significance of the dependency array in useEffect?
38. Explain the use of useReducer in React.
39. What is PropTypes in React?
40. How do you handle conditional styles in React?
41. How do you update the state based on the previous state in React?

1. What is React JS?

Answer:

React JS is an open-source JavaScript library developed by Facebook for building user interfaces, particularly for single-page applications. It allows developers to create large web applications that can change data without reloading the page. The key feature of React is its component-based architecture, which promotes reusable UI components.

React employs a declarative approach, making it easier to reason about the application’s state and how it should look at any given time. This library efficiently updates and renders the right components when data changes, thanks to its virtual DOM implementation.

2. What are the key features of React JS?

Answer:

• Component-Based Architecture: React applications are built using components, which are reusable pieces of code that manage their own state and compose to form complex UIs.
• Virtual DOM: React maintains a lightweight representation of the real DOM in memory, allowing it to perform updates more efficiently by only re-rendering components that have changed.
• JSX (JavaScript XML): JSX is a syntax extension that allows mixing HTML with JavaScript. It makes writing React components easier and more intuitive.
• Unidirectional Data Flow: Data in React flows in one direction—from parent to child—which simplifies debugging and enhances performance.
• Lifecycle Methods: React components have lifecycle methods that allow developers to hook into different stages of a component’s life, such as mounting, updating, and unmounting.

3. What is JSX? Why do we use it?

Answer:

JSX stands for JavaScript XML. It is a syntax extension for JavaScript that allows you to write HTML-like code within JavaScript files. JSX makes it easy to visualize the UI structure and is transformed into JavaScript function calls by tools like Babel before being rendered by the browser.Using JSX provides several benefits:

• Readability: It makes the code more readable and easier to understand.
• Declarative Syntax: It allows developers to describe what the UI should look like rather than how to achieve it.
• Integration with JavaScript Logic: You can embed expressions directly within JSX using curly braces {}.

4. What is the Virtual DOM? How does it work?

Answer:

The Virtual DOM is an in-memory representation of the real DOM elements generated by React components. When a component’s state changes, React first updates the Virtual DOM instead of directly manipulating the real DOM.The process works as follows:

1. Re-rendering: When state changes, React re-renders the component in the Virtual DOM.
2. Diffing: React compares the newly rendered Virtual DOM with a snapshot of the previous Virtual DOM to identify what has changed.
3. Updating Real DOM: Only those elements that have changed are updated in the real DOM, minimizing performance costs associated with direct manipulation.

This approach enhances performance because manipulating the real DOM is significantly slower compared to operations in memory.

5. Explain the difference between Real DOM and Virtual DOM.

Answer:

The Virtual DOM provides an efficient way to update the UI without incurring heavy performance costs associated with frequent updates in the Real DOM.

6. How do you create components in React?

Components can be created in two ways: Class Components and Functional Components.

Answer:

• Class Component Example:

import React from 'react';
class MyComponent extends React.Component {
render() {
   return <h1>Hello, World!</h1>;
  }
}

• Functional Component Example:

import React from 'react';
const MyComponent = () => {
   return <h1>Hello, World!</h1>;
}

Functional components are simpler and preferred for most cases due to their ease of use and ability to leverage hooks for state management and lifecycle features.

7. What are props in React?

Answer:

Props (short for properties) are a mechanism for passing data from parent components to child components in React. They are read-only and help maintain unidirectional data flow within an application.Example usage of props:

const Greeting = (props) => {
  return <h1>Hello, {props.name}!</h1>;
  }
const App = () => {
return <Greeting name="Alice" />;
}

In this example, name is passed as a prop from App to Greeting, allowing Greeting to access it via props.name.

8. What are state and lifecycle methods in React?

Answer:

State refers to a built-in object that allows components to create dynamic and interactive user interfaces by storing data that can change over time. Unlike props, which are passed down from parent components, state is managed within the component itself.Lifecycle methods are hooks provided by React that allow developers to run code at specific points in a component’s life cycle:

• componentDidMount(): Invoked immediately after a component is mounted.
• componentDidUpdate(): Invoked immediately after updating occurs.
• componentWillUnmount(): Invoked immediately before a component is unmounted and destroyed.

These methods enable you to perform operations like fetching data or cleaning up resources when components mount or unmount.

9. What is Redux and how does it relate to React?

Answer:

Redux is a predictable state container for JavaScript applications, often used with React for managing application state globally. It follows three core principles:

• Single Source of Truth: The entire application state is stored in a single store.
• State is Read-Only: The only way to change the state is by dispatching actions.
• Changes are Made with Pure Functions: Reducers specify how actions transform the state into a new state.

Redux helps manage complex application states efficiently, especially when dealing with multiple components needing access to shared state.

10. Explain how you can handle forms in React.

Answer:

Handling forms in React involves managing input values through controlled components or uncontrolled components:

• Controlled Components: The form data is handled by the component’s state via event handlers.

Example:

import React, { useState } from 'react';
const MyForm = () => {
   const [inputValue, setInputValue] = useState('');
const handleChange = (event) => {
   setInputValue(event.target.value);
}
const handleSubmit = (event) => {
event.preventDefault();
    alert('A name was submitted: ' + inputValue);
}
return (
    <form onSubmit={handleSubmit}>
       <input type="text" value={inputValue} onChange={handleChange} />
       <button type="submit">Submit</button>
       </form>
);
}

In this example, inputValue holds the current value of the input field, which updates on every change event.

11. What are hooks in React?

Answer:

Hooks are functions that let you use state and other React features without writing a class. They were introduced in React 16.8 and allow functional components to manage local component state and lifecycle features.Some commonly used hooks include:

• useState(): Allows you to add state management within functional components.
• useEffect(): Enables you to perform side effects such as fetching data or subscribing/unsubscribing from events.
• useContext(): Lets you subscribe to context changes without introducing nesting.

Example using hooks:

import React, { useState } from 'react';
   const Counter = () => {
     const [count, setCount] = useState(0);
 return (
 <div>
<p>You clicked {count} times</p>
       <button onClick={() => setCount(count + 1)}>Click me</button>
 </div>
);
}

In this example, clicking the button increments the count displayed on screen using useState.

12. What is prop drilling? How can it be avoided?

Answer:

Prop drilling refers to passing data through many layers of nested components until it reaches its intended destination component. This can lead to cumbersome code if many intermediate components do not need access to those props but must pass them along anyway.To avoid prop drilling:

• Use Context API: This allows you to share values across your component tree without having to explicitly pass props through every level.
• Use libraries like Redux or other state management solutions that provide global access to application state without needing prop drilling.

Example using Context API:

const MyContext = React.createContext();
const ParentComponent = () => {
   return (
    <MyContext.Provider value={"Hello from Context!"}>
    <ChildComponent />
    </MyContext.Provider>
  );
}
const ChildComponent = () => {
      return (
        <MyContext.Consumer>
            {value => <h1>{value}</h1>}
        </MyContext.Consumer>
);
}

In this example, ChildComponent accesses context directly without needing props passed down through intermediate components.

13. How does error handling work in React?

Answer:

Error handling in React can be managed using Error Boundaries, which are special components that catch JavaScript errors anywhere in their child component tree during rendering, lifecycle methods, and constructors.To implement an error boundary:

1. Create a class component that implements either static getDerivedStateFromError() or componentDidCatch() lifecycle methods.
2. Wrap your child components with this error boundary component.

Example:

class ErrorBoundary extends React.Component {
constructor(props) {
super(props);
this.state = { hasError: false };
}
static getDerivedStateFromError(error) {
return { hasError: true };
}
componentDidCatch(error, errorInfo) {
console.log("Error logged:", error);
}
render() {
if (this.state.hasError) {
return <h1>Something went wrong.</h1>;
}
return this.props.children;
}
}
// Usage
<ErrorBoundary>
<MyComponent />
</ErrorBoundary>

In this example, if any error occurs within MyComponent, it will be caught by ErrorBoundary, preventing crashes and allowing fallback UI rendering instead.

14. What are controlled vs uncontrolled components?

Answer:

Controlled components are those where form data is handled by the component’s state via event handlers (as shown earlier). Uncontrolled components store their own state internally and rely on refs for accessing values when needed.

Example of an uncontrolled component:

import React, { useRef } from 'react';
const UncontrolledForm = () => {
const inputRef = useRef();
const handleSubmit = (event) => {
event.preventDefault();
alert('A name was submitted: ' + inputRef.current.value);
}
return (
<form onSubmit={handleSubmit}>
<input type="text" ref={inputRef} />
<button type="submit">Submit</button>
</form>
);
}

In this case, input value is accessed directly via a ref instead of maintaining it in local component state.

15. What is routing in React? Why do we need it?

Answer:

Routing refers to navigating between different views or pages within a single-page application (SPA). In React applications, routing can be achieved using libraries such as React Router which enables dynamic routing capabilities based on URL paths.Key benefits of routing include:

• Allows users to navigate between different views seamlessly without full page reloads.
• Supports deep linking where users can bookmark specific views.
• Enables nested routes for better organization of related views/components within an app structure.

Example using React Router:

import { BrowserRouter as Router, Route } from 'react-router-dom';
const App = () => (
<Router>
<Route path="/" exact component={Home} />
<Route path="/about" component={About} />
</Router>
);

In this example, different routes render different components based on URL paths defined within <Router> tags.

16. How do you optimize performance in a React application?

Answer: Performance optimization strategies include:

• Code Splitting: Use dynamic imports or libraries like React.lazy and Suspense for lazy loading parts of your application only when needed.
• Memoization: Use React.memo for functional components or PureComponent for class-based ones to prevent unnecessary re-renders when props/state do not change.
• Use of Keys: Provide unique keys when rendering lists of elements so that React can efficiently identify which items have changed.
• Avoid Inline Functions/Objects: Define functions outside render methods or use callbacks wisely since inline definitions create new references on each render cycle leading to performance hits.

Implementing these strategies helps ensure your application remains responsive even under heavy load or complex UI interactions.

17. What is Context API? When would you use it?

Answer:

The Context API provides a way for components at different levels of an application tree to share values without having to pass props explicitly down through every level of nesting.You would use Context API when:

• You have global data that needs access across many levels/components (e.g., user authentication status).
• You want cleaner code without excessive prop drilling.

Example usage:

const ThemeContext = createContext('light');
const App = () => (
<ThemeContext.Provider value="dark">
<Toolbar />
</ThemeContext.Provider>
);
const Toolbar = () => (
<div>
<ThemedButton />
</div>
);
const ThemedButton = () => (
<ThemeContext.Consumer>
{theme => <button className={theme}>I am styled by theme context!</button>}
</ThemeContext.Consumer>
);

Here ThemedButton accesses theme context directly rather than receiving it through props from parent components like Toolbar.

18. Explain Higher Order Components (HOCs).

Answer:

Higher Order Components (HOCs) are functions that take a component as an argument and return a new component with enhanced capabilities or behavior—essentially they enable code reuse across multiple components while adhering to DRY principles (Don’t Repeat Yourself).Common uses include adding functionality like logging or conditional rendering based on props/state changes without modifying original component logic directly.

Example HOC implementation:

function withLogging(WrappedComponent) {
return class extends React.Component {
componentDidMount() {
console.log('Component mounted');
}
render() {
return <WrappedComponent {...this.props} />;
}
};
}
// Usage
const EnhancedComponent = withLogging(MyComponent);

In this case, withLogging adds logging functionality whenever MyComponent mounts while preserving its original behavior intact.

19. What are refs in React? When would you use them?

Answer:

Refs provide a way for accessing DOM nodes or instances of class components directly within functional or class-based components without relying solely on props/state management techniques which might introduce unnecessary complexity during certain operations like focusing an input field or triggering animations programmatically.You would typically use refs when:

• You need direct access/manipulation over specific elements (like focusing inputs).
• Integrating third-party libraries requiring direct interaction with DOM nodes.

Example usage of refs:

import { useRef } from 'react';
const InputFocusExample = () => {
const inputRef = useRef(null);
const focusInput = () => {
inputRef.current.focus();
};
return (
<>
<input ref={inputRef} type="text" />
<button onClick={focusInput}>Focus Input</button>
</>
);
};

Here clicking “Focus Input” button triggers focus action on input element via ref reference.

20. What are synthetic events in React?

Answer:

Synthetic events are objects created by React as wrappers around native browser events providing consistent behavior across all browsers while also enabling additional functionality like pooling events for performance optimization purposes.Key characteristics include:

• Synthetic events normalize events across browsers ensuring compatibility regardless of differences between implementations.
• They implement event delegation allowing single event listeners at higher levels rather than attaching them individually per element.
• They automatically clean up after themselves reducing memory overhead during rapid interactions.

Example usage:

const ButtonClickExample = () => {
const handleClick = (event) => {
console.log(event); // SyntheticEvent object
};
return (
<button onClick={handleClick}>Click Me!</button>
);
};

In this case clicking button logs synthetic event object containing relevant information about click action.

Here’s an additional list of top 20+ React JS interview questions with detailed answers for freshers, designed to complement the sample questions:

21. What is the purpose of keys in React?

Answer:

Keys help React identify which items have changed, been added, or removed. They should be unique among siblings to ensure that React can update only the necessary components, optimizing re-rendering.

22. What is a React fragment, and why would you use it?

Answer:

React fragments (<></> or <React.Fragment>) allow you to group multiple elements without adding extra nodes to the DOM. This helps avoid unnecessary nesting and improves performance.

23. How do you handle events in React?

Answer:

Event handling in React is done using camelCase syntax (e.g., onClick) and passing a function as the event handler. React events are synthetic events that wrap native browser events for cross-browser compatibility.

24. Explain React’s reconciliation process.

Answer:

React’s reconciliation process involves comparing the new virtual DOM with the previous one to identify changes. React then updates only the changed parts of the real DOM, which improves performance.

25. What is conditional rendering in React? How do you implement it?

Answer:

Conditional rendering in React lets you render different components based on a condition, typically implemented using JavaScript operators like if, &&, ? :, or switch statements.

26. What is the purpose of the useEffect hook?

Answer:

useEffect lets you perform side effects in function components, such as fetching data, directly interacting with the DOM, or subscribing to events. It runs after every render by default and can be customized with dependencies.

27. How can you make an HTTP request in a React app?

Answer:

HTTP requests in React can be made using libraries like Axios or the Fetch API, typically within lifecycle methods or useEffect to fetch data when the component mounts or updates.

28. What is the purpose of the useState hook?

Answer:

useState is a hook that lets you add state to functional components. It returns the current state and a function to update that state.

29. What are controlled and uncontrolled inputs in React?

Answer:

In controlled inputs, React state is the single source of truth for input values, while uncontrolled inputs store their own state and rely on refs for accessing values. Controlled inputs offer better state management.

30. Explain the concept of memoization in React.

Answer:

Memoization in React is a technique to optimize performance by caching results of expensive computations. React.memo is used to prevent re-rendering of functional components if their props haven’t changed.

31. How does React handle updates in child components?

Answer:

React re-renders child components whenever their parent re-renders, unless the child components are optimized with techniques like React.memo or shouldComponentUpdate.

32. What is lazy loading in React, and how is it implemented?

Answer:

Lazy loading delays loading of components until they are needed, improving performance. In React, React.lazy combined with Suspense allows you to load components on demand.

33. How do you pass data between sibling components in React?

Answer:

Data can be passed between sibling components by lifting state up to a common ancestor component or using the Context API to share state more broadly.

34. What is code-splitting in React?

Answer:

Code-splitting is a technique for splitting your code into smaller chunks, typically done with React.lazy and Suspense. It helps in loading only the necessary code when needed, reducing initial load times.

35. How do you handle errors in a React component?

Answer:

Error boundaries (componentDidCatch in class components) catch errors in rendering, lifecycle methods, and constructors of child components. For functional components, try-catch blocks within useEffect can handle asynchronous errors.

36. What are default props in React?

Answer:

Default props are used to set default values for props if they are not passed by the parent component. In functional components, they can be set using Component.defaultProps.

37. What is the significance of the dependency array in useEffect?

Answer:

The dependency array determines when useEffect should re-run. If empty, it runs only once after the initial render. If dependencies are provided, it runs whenever any dependency value changes.

38. Explain the use of useReducer in React.

Answer:

useReducer is a hook for managing complex state logic by dispatching actions to a reducer function, similar to Redux. It’s useful for handling state transitions and logic-heavy state updates in components.

39. What is PropTypes in React?

Answer:

PropTypes is a type-checking library for React props. It helps validate that props passed to components match expected types, improving debugging and preventing runtime errors.

40. How do you handle conditional styles in React?

Answer:

Conditional styles in React can be applied using inline styles with JavaScript conditions, CSS classes (using conditional className), or styled-components for more dynamic styling based on props or state.

41. How do you update the state based on the previous state in React?

Answer:

To update state based on the previous state, you pass a callback function to the setState function or useState update function. This ensures you’re working with the latest state value, which is crucial in asynchronous updates.

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