React JS Top 100 Interview Questions and Answers – Part 2

Welcome back to our comprehensive React JS Top 100 interview questions and answers guide.

In the first part of this React JS Top 100 Interview Questions and Answers series, we delved into the foundational aspects of React, covering essential topics such as components, state, and props.

As we continue our journey in Part 2, we will explore more advanced concepts and techniques that are crucial for any React developer aiming to excel in job interviews.

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

• The useContext hook is used to consume values from the React context without nesting multiple components. 
• It provides a cleaner way to access global data or shared state across the component tree.

26. What are portals in React and when would you use them?

• Portals provide a way to render children components into a different DOM node outside the parent component’s DOM hierarchy. 
• They are useful for scenarios such as modals, tooltips, or any component that needs to break out of its container.

27. What is the purpose of the useLayoutEffect hook in React?

• The useLayoutEffect hook is similar to useEffect, but it fires synchronously after all DOM mutations. 
• It’s useful for tasks that require DOM measurements or other imperative mutations before the browser paints.

28. Explain the concept of code splitting in React.

• Code splitting is a technique used to split the code into smaller chunks, allowing parts of the application to be loaded asynchronously at runtime. 
• This improves initial loading performance by reducing the size of the initial bundle and only loading code when it’s needed.

29. How do you perform server-side rendering (SSR) in React?

• Server-side rendering (SSR) is the process of rendering React components on the server and sending the pre-rendered HTML to the client. 
• This improves initial load performance and enables search engine optimization (SEO) by providing fully rendered content to web crawlers.

Server-side code is given below

In the client-side code is given below

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

• The useCallback hook is used to memoize functions in functional components to prevent unnecessary re-renders. 
• It’s particularly useful for optimizing performance when passing callbacks to child components that rely on reference equality.

31. What is the purpose of the useImperativeHandle hook in React?

• The useImperativeHandle hook is used to customize the instance value that is exposed when using ref with functional components. 
• It allows you to specify which values should be exposed to parent components when using ref, providing more control over the ref’s interface.

32. Explain the concept of reconciliation in React.

• Reconciliation is the process by which React updates the DOM to match the virtual DOM representation of components.
  
• It involves comparing the current virtual DOM with the previous one and applying the minimal set of changes necessary to reflect the component updates efficiently.

33. What is the purpose of the useEffect hook’s dependency array?

• The dependency array in the useEffect hook specifies the values (dependencies) that the effect depends on.
  
• If any of the values in the dependency array change between renders, the effect will re-run.
  
• Omitting the dependency array or passing an empty array means the effect runs only once after the initial render.

34. Explain the concept of context in React and when you would use it?

• Context provides a way to share data between components without having to pass props manually at every level of the component tree.
   
• It’s useful for passing global data such as user authentication, theme, or localization preferences to deeply nested components.

35. How do you update the state in React?

• State can be updated in React using the useState hook’s setter function or by calling setState method in class components.
   
• It’s important to remember that state updates are asynchronous, and React may batch multiple state updates for performance reasons.

36. What is the purpose of the componentDidCatch lifecycle method in React?

• The componentDidCatch lifecycle method is invoked when an error occurs in a component’s child tree during rendering, lifecycle methods, or in the constructor.
   
• It allows the component to catch and handle errors gracefully, preventing the entire application from crashing.

37. Explain the concept of keys in React lists and why they are important.

• Keys are special attributes used by React to identify which items have changed, added, or removed in a list. 
• They help React efficiently update the DOM by maintaining component state and preventing unnecessary re-renders when the list changes.

38. How do you prevent a component from rendering in React?

You can prevent a component from rendering in React by using conditional rendering techniques such as returning null, rendering a placeholder component, or using React’s built-in conditional rendering features like ternary operators or logical && operator.

39. What are the benefits of using React Router?

• React Router is a popular library for handling routing in React applications.
  
• It provides a declarative way to define routes and navigate between different views.
  
• Some benefits of using React Router include code splitting, nested routes, route-based code splitting, and easy integration with React components.

40. Explain the concept of memoization in React.

• Memoization is the process of caching the result of expensive function calls and returning the cached result when the same inputs occur again.
  
• In React, memoization can be achieved using the useMemo and useCallback hooks to optimize performance by avoiding unnecessary re-renders or recalculations.

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

• The useState hook is used to add state to functional components in React. 
• It returns a stateful value and a function to update that value, allowing you to manage state without writing a class component.

42. How do you perform AJAX requests in React?

AJAX requests in React can be performed using the Fetch API or third-party libraries such as Axios. Here’s an example using the Fetch API:

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

• The useEffect hook is used to perform side effects in functional components.
  
• It’s similar to lifecycle methods in class components and allows you to execute code in response to component mounts, updates, and unmounts.

44. What is the difference between useEffect and useLayoutEffect hooks in React?

• The useEffect hook runs asynchronously after the DOM has been updated, while the useLayoutEffect hook runs synchronously after all DOM mutations.
   
• In most cases, you should use useEffect, but if your effect requires measurements or mutations that affect the layout, you should use useLayoutEffect.

45. How do you optimize performance in React using memoization?

• Memoization in React can be achieved using the useMemo and useCallback hooks.
  
• They memoize the result of expensive computations or functions, preventing unnecessary re-renders and improving performance.

46. Explain the concept of PureComponent in React.

• PureComponent is a class component provided by React that implements shouldComponentUpdate with a shallow prop and state comparison. 
• It prevents unnecessary re-renders by automatically performing a shallow comparison of props and state.

47. What is the purpose of the createContext function in React?

• The createContext function is used to create a React context, which provides a way to pass data through the component tree without having to pass props manually at every level. 
• It consists of two components: Provider and Consumer, which allow data to be shared across multiple components.

48. How do you handle routing in React applications?

• Routing in React applications can be handled using libraries such as React Router.
  
• React Router provides a declarative way to define routes and navigate between different views in a single-page application.

49. What are controlled components in React?

• Controlled components are form elements whose value is controlled by React state.
  
• Their value is set by the state and updated through event handlers, ensuring that React is the single source of truth for the component’s state.

50. Explain the purpose of the forwardRef function in React.

• The forwardRef function is used to forward refs from a child component to a DOM element or a class component passed as props.
   
• It allows components to pass refs through intermediate components without breaking the component’s encapsulation.

51. What is the purpose of the React.StrictMode component in React?

• The React.StrictMode component is used to enable strict mode, which performs additional checks and warnings to help identify potential problems in your application.
   
• It’s particularly useful for highlighting deprecated lifecycle methods, unsafe legacy lifecycles, and other potential issues.

52. Explain the purpose of the useContext hook in React.

• The useContext hook is used to consume values from the React context without nesting multiple components. 
• It provides a cleaner way to access global data or shared state across the component tree.

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

• The useReducer hook is used to manage complex state logic in functional components.
  
• It is an alternative to useState and is particularly useful when the next state depends on the previous one and requires more complex updates.

54. What is the purpose of the React.memo function in React?

The React.memo function is a higher-order component used to memoize the result of a functional component, preventing unnecessary re-renders when the component’s props have not changed.

55. What is the purpose of the React.Fragment component in React?

• The React.Fragment component is used to group multiple children elements without adding extra nodes to the DOM. 
• It’s similar to using empty angle brackets (<>…</>) as a shorthand for creating fragments.

The React JS Interview questions and answer series is continued in part-3.

Please make sure you go through all the parts to learn and crack the react interview.

Good luck.