Top 40+ React Coding Interview Questions and Answers

11. What is the Purpose of useMemo?

The useMemo hook is used to optimize performance in functional components by memoizing expensive calculations. It returns a memoized value that is recalculated only when its dependencies change, preventing unnecessary recalculations during re-renders, which is beneficial for performance, especially with complex computations.

12. What is Prop Drilling?

Prop drilling occurs when data is passed down through multiple layers of components, which can complicate the code and make it harder to manage. This often leads to a situation where a component needs to pass value in proportions to its child components just to get data to a deeper nested component. The Context API can help to solve these issues by allowing data to be shared directly among components without the need to pass react components through an interface.

13. How do you Achieve Lazy Loading in React?

Lazy loading can be implemented using react.lazy and Suspense to load components only when they are needed, improving initial load time and performance.

14. What are Fragment Components?

Fragments allow developers to group several elements without inserting extra nodes into the DOM. This helps to keep the DOM tree clean and optimize rendering.

15. How Do You Handle API Calls in React?

API calls are normally done using the useEffect() hook to get data when a component mounts. This makes the component reflect the latest data from the server and cause updates depending on user interaction or state change. Async/await in useEffect() helps it to be easier to have cleaner, readable code if there is asynchronous action.

16. What are React DevTools?

React DevTools is an extension for the browser that gives developers a means of viewing the React tree of components, optimizing the performance, and debugging the application more effectively. It enables one to view component props and state, profile the component rendering, and see updates, thus being a very useful tool in React development.

17. What is the Importance of the React Key Prop?

Primary key is required to assist React in determining which elements in a list have been added, updated, or deleted. Keys should be unique from one another and immutable so that it can offer optimal updates. Proper key usage optimizes performance by preventing unnecessary rendering so that React can efficiently handle DOM changes.

18. What is a Higher-Order Component (HOC)?

A higher-order component (HOC) is a function that returns a new component with additional behavior after passing a component as an argument. HOCs are used for a variety of concerns like fetching data, authentication, or logging. The pattern promotes code reuse since frequent behavior is separated from specific components.

19. What is the Purpose of useReducer()?

The useReducer() hook is utilized for handling complicated logic in functional components. It is similar to Redux. The hook is very handy to handle state changes on the basis of various values or having intricate logic. With useReducer, one can have a properly organized and deterministic state management.

20. What is the Use of useMemo?

The useMemo hook is utilized to memoize expensive calculations and avoid re-renders when recalculations are unnecessary and improve performance.

21. Why is the React Key Prop necessary?

The key prop is needed so that React can be notified about what things in a list have changed so that it can update them correctly and avoid unnecessary rendering.

22. Why is React Fragment?

React Fragment helps you to render a group of children without adding extra nodes to the DOM. It prevents unnecessary wrapper nodes, which keeps the markup cleaner and performance improved. You can also use <>.</> syntax for fragments.

23. What are PropTypes in React?

PropTypes is one of the methods of validating prop types used when passing to a component. They prevent bugs by having correct data types and error messages during development mode.

24. What is the difference between React and Angular?

Following is the comparison of React and Angular:

• Angular is built by Google, whereas React is built by Facebook.
• Angular is an entire MVC framework, whereas React operates using the view layer.
• Angular utilizes two-way data binding, whereas React utilizes one-way data binding.

25. How can I comment in React?

You can comment in React in two ways:

• Single-line comments: // This is a comment
• Multi-line comments: {/* This is a multi-line comment */}

26. . What do you mean by an arrow function, and how is it used in React?

In Javascript, an arrow function is a concise way to write functions. In React, they are particularly useful because they do not bind their own, and preventing issues in callbacks.

27. How is React different from React Native?

Here is the comparison of react and react native:

• React is for web applications, while React Native is for mobile applications (iOS and Android).
• React uses HTML, whereas React Native uses native components.
• React uses CSS, while React Native uses a styling system that resembles CSS but with different properties.

28. Explain how lists work in React.

Lists in React are created using the map() function to iterate over an array of data, generating a set of elements. Each element in the list should have a unique key prop to help React identify and manage updates efficiently.

29. Why is there a need for using keys in lists?

Keys are essential for identifying which items in a list have changed, been added, or removed. They help React optimize rendering by allowing it to re-render only the changed components rather than the entire list.

30. What are synthetic events in React?

Synthetic events are React’s cross-browser wrapper for native events. They provide a consistent API and help in normalizing event properties across different browsers, ensuring compatibility.

31. How do you create a React app?

• Install Node.js to use npm.
• Install the create-react-app package via the command prompt.
• Set up a text editor like VS Code or Sublime Text.
• Create a new app using the command npx create-react-app my-app.

32. How do you create an event in React?

You create an event in React by defining an event handler function and attaching it to an element using JSX. For example:

<button onClick={this.handleClick}>Click me</button>

Here are the advanced react interview questions asked in interviews:

33. What is the Difference Between Controlled and Uncontrolled Components?

Controlled components depend on the state to manage their values, meaning that form inputs are controlled by React's state. This approach provides better data flow, validation, and easier management of form state. While uncontrolled components store their values in the DOM, allowing the input elements to maintain their state.

34. How to modify the counter to include a reset button that sets the count back to zero?

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

35. Explain the difference between Props and State.

Props are read-only data passed from parent components to child components, allowing for data flow and communication. While the state is mutable and managed within a component. It is used to track local component data that can change over time, triggering re-renders when updated.

36. How to implement a feature to mark a to-do item as completed and style it differently?

function TodoApp() {
  const [todos, setTodos] = useState([]);
  const [input, setInput] = useState("");

  const addTodo = () => {
    setTodos([...todos, input]);
    setInput("");
  };

  return (
    <div>
      <input value={input} onChange={(e) => setInput(e.target.value)} />
      <button onClick={addTodo}>Add</button>
      <ul>
        {todos.map((todo, index) => (
          <li key={index}>{todo}</li>
        ))}
      </ul>
    </div>
  );
}

37. How to handle authentication in a React application?

Authentication can be managed using context or state management libraries such as Redux to store user data. Higher-order components are used to wrap protected routes, authenticating before rendering the components. Using JWT tokens in API requests is also standard procedure.

38. What is the difference between static and dynamic routing in React Router?

Static routing refers to predefined routes that do not change, while dynamic routing allows routes to be defined at runtime based on data. Dynamic routing can be achieved using route parameters and can change based on application state or API responses.

39. What is the React Synthetic Event System?

React's synthetic event system is a cross-browser wrapper around the browser's native event system. It normalizes events so that they have consistent properties across different browsers. Synthetic events are pooled for performance, meaning that their properties are reused, which is why you cannot access event properties asynchronously without calling event.persist().

40. How to handle loading and error states when fetching data from the API?

function FetchData() {
  const [data, setData] = useState([]);

  useEffect(() => {
    fetch("https://jsonplaceholder.typicode.com/posts")
      .then(response => response.json())
      .then(data => setData(data));
  }, []);

  return (
    <ul>
      {data.map(item => (
        <li key={item.id}>{item.title}</li>
      ))}
    </ul>
  );
}

41. What are custom hooks, and how are they created?

Custom hooks are JavaScript functions that use React hooks and encapsulate reusable logic. They can take arguments and return values. You create a custom hook by defining a function that starts with "use" and inside it, you can use other hooks as needed. For example, function useFetch(url) { ... }.

The following are some of the best tips that can ensure you excel in your react coding interview questions:

• Ensure that you have a firm understanding of the fundamental concepts of React and its fundamental ideas.
• Practice react js coding interview questions and coding interview challenges in react on platforms such as LeetCode or CodeChef.
• Develop mini-projects to reinforce your grasp of concepts and demonstrate your proficiency.
• Study advanced topics and react js technical interview questions to be different.
• Practice mock interviews to build your confidence and delivery.

In conclusion, react coding interviews can be challenging, but thorough preparation and practice can significantly increase your chances of success. Focus on understanding both basic and advanced concepts, and don’t hesitate to build projects that highlight your skills.