React Interview Questions

1. What is React, and what are its key features?

React is an open-source JavaScript library developed by Facebook for building user interfaces, particularly single-page applications (SPAs). It allows developers to create reusable UI components and efficiently update the DOM using a virtual DOM. Key features of React include a component-based architecture, declarative syntax, and unidirectional data flow. React also supports server-side rendering (SSR) through frameworks like Next.js, enhancing performance and SEO. Its ecosystem includes tools like React Router for navigation, Redux for state management, and React Native for mobile app development. React’s simplicity, flexibility, and performance make it a popular choice for modern web development.

2. What is the difference between React and Angular?

React and Angular are both popular frameworks for building web applications, but they differ in architecture and approach. React is a library focused on the view layer, offering flexibility and a lightweight solution for building UIs. Angular, on the other hand, is a full-fledged framework with built-in features like dependency injection, routing, and form validation. React uses JSX for templating, while Angular uses HTML with directives. React’s virtual DOM provides better performance for dynamic updates, whereas Angular’s two-way data binding simplifies synchronization between the model and view. React is more suitable for SPAs, while Angular is ideal for enterprise-level applications.

3. What are React components, and how are they created?

React components are the building blocks of a React application. They are reusable, self-contained pieces of code that define the structure and behavior of a part of the UI. Components can be created as functional components or class components. Functional components are simpler and use JavaScript functions, while class components use ES6 classes and have additional features like lifecycle methods. For example:

function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

Components can receive data via props and manage internal state using hooks (in functional components) or this.state (in class components). They promote reusability and modularity in React applications.

4. What is JSX, and why is it used in React?

JSX (JavaScript XML) 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 of components in a more readable and expressive way. For example:

const element = <h1>Hello, World!</h1>;

JSX is not mandatory in React, but it simplifies the creation of complex UI structures. Under the hood, JSX is transpiled into JavaScript using tools like Babel. It enables developers to combine the power of JavaScript with the simplicity of HTML, making React code more intuitive and maintainable.

5. What is the difference between state and props in React?

State and props are both used to manage data in React, but they serve different purposes. Props (short for properties) are read-only and passed from a parent component to a child component. They are used to customize and configure components. For example:

function Welcome(props) {
  return <h1>Hello, {props.name}</h1>;
}

State, on the other hand, is mutable and managed within a component. It is used to store data that can change over time, such as user input or API responses. For example:

class Counter extends React.Component {
  state = { count: 0 };
  render() {
    return <div>{this.state.count}</div>;
  }
}

Understanding the difference between state and props is crucial for effective React development.

6. What are React hooks, and why are they used?

React hooks are functions that allow you to use state and other React features in functional components. They were introduced in React 16.8 to address limitations of class components, such as complex lifecycle methods and code reuse. Common hooks include useState for managing state, useEffect for side effects, and useContext for accessing context. For example:

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>{count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

Hooks simplify code, improve readability, and promote reusability, making them a cornerstone of modern React development.

7. What is the purpose of the useState hook in React?

The useState hook in React is used to add state to functional components. It returns an array with two elements: the current state value and a function to update it. For example:

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>{count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

useState allows functional components to manage dynamic data without converting them to class components. It is one of the most commonly used hooks in React.

8. What is the purpose of the useEffect hook in React?

The useEffect hook in React is used to perform side effects in functional components, such as fetching data, updating the DOM, or subscribing to events. It runs after every render by default but can be controlled using a dependency array. For example:

function DataFetcher() {
  const [data, setData] = useState(null);
  useEffect(() => {
    fetch('/api/data')
      .then(response => response.json())
      .then(data => setData(data));
  }, []);
  return <div>{data ? data.message : 'Loading...'}</div>;
}

useEffect replaces lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount in class components, making it a powerful tool for managing side effects.

9. What is the purpose of the useContext hook in React?

The useContext hook in React is used to access the value of a context without wrapping components in a Context.Consumer. It simplifies the process of sharing data across the component tree. For example:

const ThemeContext = React.createContext('light');
function ThemedButton() {
  const theme = useContext(ThemeContext);
  return <button style={{ background: theme }}>Click me</button>;
}

useContext is particularly useful for global state management, such as themes, user authentication, or language preferences.

10. What is the purpose of the useReducer hook in React?

The useReducer hook in React is used to manage complex state logic in functional components. It is an alternative to useState and is inspired by Redux. It takes a reducer function and an initial state as arguments and returns the current state and a dispatch function. For example:

function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    default:
      throw new Error();
  }
}
function Counter() {
  const [state, dispatch] = useReducer(reducer, { count: 0 });
  return (
    <div>
      <p>{state.count}</p>
      <button onClick={() => dispatch({ type: 'increment' })}>Increment</button>
    </div>
  );
}

useReducer is ideal for managing state transitions in a predictable and scalable way.

11. What is the purpose of the useRef hook in React?

The useRef hook in React is used to create a mutable reference that persists across renders. It is commonly used to access DOM elements or store values that do not trigger re-renders. For example:

function TextInput() {
  const inputRef = useRef(null);
  const focusInput = () => inputRef.current.focus();
  return (
    <div>
      <input ref={inputRef} type="text" />
      <button onClick={focusInput}>Focus Input</button>
    </div>
  );
}

useRef is useful for interacting with the DOM, managing timers, or storing previous values.

12. What is the purpose of the useMemo hook in React?

The useMemo hook in React is used to memoize expensive calculations and optimize performance. It returns a memoized value that is recalculated only when its dependencies change. For example:

function ExpensiveComponent({ a, b }) {
  const result = useMemo(() => computeExpensiveValue(a, b), [a, b]);
  return <div>{result}</div>;
}

useMemo prevents unnecessary recalculations and improves the efficiency of React applications.

13. What is the purpose of the useCallback hook in React?

The useCallback hook in React is used to memoize functions and prevent unnecessary re-renders. It returns a memoized version of the function that only changes when its dependencies change. For example:

function ParentComponent() {
  const [count, setCount] = useState(0);
  const increment = useCallback(() => setCount(count + 1), [count]);
  return <ChildComponent onClick={increment} />;
}

useCallback is useful for optimizing performance in components that rely on callback functions.

14. What is the purpose of the React.memo function?

The React.memo function is used to memoize functional components and prevent unnecessary re-renders. It performs a shallow comparison of props and re-renders the component only if the props change. For example:

const MemoizedComponent = React.memo(function MyComponent(props) {
  return <div>{props.value}</div>;
});

React.memo is a performance optimization tool that reduces redundant renders in React applications.

15. What is the purpose of the key prop in React?

The key prop in React is used to uniquely identify elements in a list. It helps React efficiently update the DOM by tracking changes to individual elements. For example:

const items = ['Apple', 'Banana', 'Cherry'];
const list = items.map((item, index) => <li key={index}>{item}</li>);

Using a unique key for each element ensures that React can correctly add, remove, or reorder elements without causing performance issues.

Certainly! Here are the next set of React interview questions and detailed answers:

16. What is the purpose of the Fragment in React?

The Fragment in React is used to group multiple elements without adding an extra node to the DOM. It is particularly useful when you need to return multiple elements from a component but don’t want to wrap them in a parent div. For example:

function MyComponent() {
  return (
    <React.Fragment>
      <h1>Title</h1>
      <p>Description</p>
    </React.Fragment>
  );
}

You can also use the shorthand syntax <>...</> for fragments. Fragments improve the structure and performance of React applications by reducing unnecessary DOM nodes.

17. What is the purpose of the Portal in React?

The Portal in React is used to render a component outside its parent DOM hierarchy. It is commonly used for modals, tooltips, or dropdowns that need to be positioned independently of the parent component. For example:

function Modal({ children }) {
  return ReactDOM.createPortal(
    <div className="modal">{children}</div>,
    document.getElementById('modal-root')
  );
}

Portals allow you to break out of the current DOM tree and render components in a different part of the document, improving flexibility and accessibility.

18. What is the purpose of the Error Boundary in React?

The Error Boundary in React is a component that catches JavaScript errors in its child components and displays a fallback UI instead of crashing the entire application. It is implemented using the componentDidCatch lifecycle method. For example:

class ErrorBoundary extends React.Component {
  state = { hasError: false };
  componentDidCatch(error, info) {
    this.setState({ hasError: true });
    console.error(error, info);
  }
  render() {
    if (this.state.hasError) {
      return <h1>Something went wrong.</h1>;
    }
    return this.props.children;
  }
}

Error boundaries are essential for improving the robustness and user experience of React applications.

19. What is the purpose of the Context API in React?

The Context API in React is used to share data across the component tree without passing props manually at every level. It is particularly useful for global data like themes, user authentication, or language preferences. For example:

const ThemeContext = React.createContext('light');
function App() {
  return (
    <ThemeContext.Provider value="dark">
      <Toolbar />
    </ThemeContext.Provider>
  );
}
function Toolbar() {
  return <ThemedButton />;
}
function ThemedButton() {
  const theme = useContext(ThemeContext);
  return <button style={{ background: theme }}>Click me</button>;
}

The Context API simplifies state management and reduces prop drilling in large applications.

20. What is the purpose of the React Router in React?

The React Router is a library used for navigation and routing in React applications. It allows you to define routes and render components based on the current URL. For example:

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

React Router supports features like nested routes, route parameters, and programmatic navigation, making it essential for building SPAs.

21. What is the purpose of the Redux library in React?

The Redux library is used for state management in React applications. It provides a centralized store to manage the application’s state and enables predictable state updates using actions and reducers. For example:

const initialState = { count: 0 };
function reducer(state = initialState, action) {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    default:
      return state;
  }
}
const store = createStore(reducer);
store.dispatch({ type: 'increment' });

Redux is particularly useful for large applications with complex state logic, as it simplifies debugging and testing.

22. What is the purpose of the Thunk middleware in Redux?

The Thunk middleware in Redux is used to handle asynchronous actions, such as API calls. It allows you to dispatch functions instead of plain objects, enabling side effects like fetching data. For example:

function fetchData() {
  return dispatch => {
    dispatch({ type: 'FETCH_REQUEST' });
    fetch('/api/data')
      .then(response => response.json())
      .then(data => dispatch({ type: 'FETCH_SUCCESS', payload: data }))
      .catch(error => dispatch({ type: 'FETCH_FAILURE', error }));
  };
}

Thunk is essential for managing asynchronous operations in Redux applications.

23. What is the purpose of the Saga middleware in Redux?

The Saga middleware in Redux is used to handle side effects, such as asynchronous actions, using generator functions. It provides a more structured and testable way to manage complex workflows. For example:

function* fetchData() {
  try {
    const data = yield call(fetch, '/api/data');
    yield put({ type: 'FETCH_SUCCESS', payload: data });
  } catch (error) {
    yield put({ type: 'FETCH_FAILURE', error });
  }
}

Saga is ideal for applications with complex asynchronous logic, such as long-running tasks or race conditions.

24. What is the purpose of the Reselect library in Redux?

The Reselect library in Redux is used to create memoized selectors for efficiently computing derived data from the Redux store. It prevents unnecessary recalculations and improves performance. For example:

const getItems = state => state.items;
const getFilter = state => state.filter;
const getFilteredItems = createSelector(
  [getItems, getFilter],
  (items, filter) => items.filter(item => item.includes(filter))
);

Reselect is particularly useful for optimizing performance in large Redux applications.

25. What is the purpose of the React Testing Library?

The React Testing Library is a testing utility for React that encourages testing components in a way that resembles how users interact with them. It focuses on testing behavior rather than implementation details. For example:

import { render, screen } from '@testing-library/react';
import MyComponent from './MyComponent';
test('renders title', () => {
  render(<MyComponent />);
  expect(screen.getByText('Hello, World!')).toBeInTheDocument();
});

React Testing Library promotes best practices for writing maintainable and reliable tests in React applications.