Top 101 React Interview Questions [2024]

React Interview Questions and Answers for Freshers

Starting with fundamental questions for beginners, we will progressively cover more advanced React interview questions.

1. What Is React?

Designed for building web user interfaces (UIs), React is a JavaScript library that is declarative and component-based, allowing developers to create reusable UI components. It uses the Virtual DOM approach to enhance rendering performance by minimizing DOM updates. React is recognized for its speed and compatibility with other tools and libraries.

2. What Are the Benefits of React for Front-End Development?

This is one of the most commonly asked React interview questions. React offers several benefits that make it stand out:

• Speed: React allows developers to work independently on an application's parts, speeding up the development process. Changes made to one part won't affect the logic of the application.
• Flexibility: React's modular structure makes the code easier to maintain and flexible, which saves businesses time and money.
• Performance: React was designed for high performance. It uses a virtual DOM and server-side rendering, which makes complex apps run extremely fast.
• Ease of Use: React is easy to learn and use, especially if you have a basic knowledge of JavaScript. An experienced JavaScript developer can learn React in a day or two.

3. What Are the Limitations of React?

Following are some of the limitations of React:

• Lack of Proper Documentation: Many tools and libraries can enhance React's performance. React itself undergoes regular updates. Unfortunately, this rapid evolution comes with a downside. Some developers feel that React technologies are advancing so quickly that proper documentation and instructions cannot keep up. As a result, developers are often left with sparse text guides that lack detailed information.
• JSX Complexity: JSX is advantageous as it makes code more readable and clean. It can also be a stumbling block for those unfamiliar with it. JSX, a JavaScript extension, blends HTML and JavaScript, adding complexity to React and potentially deterring aspiring developers. Both developers and designers often find JSX challenging, leading to a steep learning curve.
• Problems With SEO: There have been concerns about Google’s ability to index dynamic websites that use client-side rendering. This issue demands significant expertise from developers, as improper implementation can result in poor SEO performance. If not handled correctly, getting a site properly indexed by search engines can lead to difficulties.

4. What Are the Major Features of React?

Here are some of React's major features:

• Building with Components: React lets you break down your user interface into small, reusable components. Each component can manage its data through state and props.
• JSX Syntax: You can write HTML-like markup inside your JavaScript code using JSX syntax. It makes React components much easier to read and write.

• Virtual DOM: Instead of updating the real DOM directly, React creates a virtual copy in memory first. When something changes, React figures out the minimum updates needed and makes those efficiently.
• One-Way Data Flow: Data in React flows down from parents to children. Child components can't directly modify data in the parent, but they can communicate upwards to trigger state changes.
• Speedy Performance: React apps can run blazingly fast by using the virtual DOM and only updating the parts that change, especially as you build more components.
• Reusable Components: React encourages building self-contained, reusable components that encapsulate their logic and styling. This modularity makes code easier to maintain.
• Single-Page Apps: React is a popular choice for modern single-page apps that load once and then smoothly update content as needed without full reloads.

5. What Is JSX?

JSX is a syntax extension for JavaScript that allows developers to write HTML-like markup directly inside your JavaScript files.

6.What Is the Difference Between an Element and a Component?

When facing a React interview, this is one of the most commonly asked React interview questions. Here are the differences between elements and components:

7. How Do You Create Components in React?

You can create components in React in two ways: functional or class components.

• Functional Components: Functional components are the easiest way to create components in React. They are plain JavaScript functions that accept props as arguments and return a React element (JSX).

Example:


• Class Components: Class components are slightly more complex than functional components. They are ES6 classes that extend the React. Component class and have access to lifecycle methods and state management.

Example:

8. What Are the Differences Between Functional and Class Components?

Here's the differences between Functional Components and Class Components in React:

9. Why Is It Considered Best Practice To Use Functional Components Over Class Components?

The reasons to use functional components over class components in React because of the following points:

• Simplicity: Functional components are simple functions with no complex class syntax or methods. They don’t have the added complexity of lifecycle methods, state management, or this binding.
• Speed: Functional components are generally faster than class components. They don't need to create a new instance every time they render, so there's less overhead. Plus, with React Hooks, functional components can do just as much without any speed issues.
• Testing: Functional components are just functions that are easy to test using standard JavaScript testing tools like Jest. You don't need to mock class instances or lifecycle methods.
• Reusability:Functional components are super easy to reuse across your app. You can move them around and use them wherever you need them.
• Hooks: It allows functional components to handle state, lifecycle events, and anything class components could do, but in a cleaner, more intuitive way.

10. What Is a State in React?

A state is an object that represents a component's current condition or properties. It allows components to create and manage their data, determining how the components will render and behave. The state is mutable and can be updated in response to user actions, API calls, or other events.

11. What Are Props in React?

Props pass data from a parent component to a child component in React. They are essentially input parameters or arguments that allow components to receive and use data from their parent or other components.

Props are immutable, meaning that the component cannot modify them once passed to a component. Instead, props must be changed by the parent component that initially passed them down.

12. What Is the Difference Between State and Props?

The main differences between state and props in React are:

13. What Is Virtual DOM?

The virtual DOM is a programming concept that maintains an ideal, or virtual, representation of a UI in memory. This virtual representation is then synchronized with the real DOM using a library like ReactDOM.

14. How Does Virtual DOM Work?

The Virtual DOM in React operates employing a process called reconciliation, involving the following steps:

• Render: When a React component's state or props change, React creates a new Virtual DOM tree representation of the updated component hierarchy.
• Diff: React then compares (or "diffs") the new Virtual DOM tree with the previous Virtual DOM tree to identify the minimum changes required to update the actual DOM.
• Batch Updates: React batches (or collects) the necessary changes into a single update operation. This batching process helps improve performance by reducing the number of direct DOM operations.
• Reconciliation: React applies the batched updates to the actual DOM by creating, updating, or removing the necessary DOM nodes.
• Re-render: React re-renders the components with the updated DOM after the reconciliation process.

15. Is the Shadow DOM the Same As the Virtual DOM?

No, they are different. The Shadow DOM is a browser technology primarily used for scoping variables and CSS within web components. In contrast, the virtual DOM is a concept implemented by JavaScript libraries on top of browser APIs to optimize UI rendering and performance.

16. What Is the Difference Between Shadow DOM and Virtual DOM?

This is yet another commonly asked React interview question. Here are the main differences between them:

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17. What Are Stateless Components?

Stateless components, functional or presentational components, do not manage or track state changes. Instead, they receive data via properties and display it. These components are functions rather than classes.

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

18. What Are Stateful Components?

Stateful components, called class or container components, store information about their state in memory. They track changes to this state and re-render when necessary. In addition to managing their state, stateful components can access React's lifecycle methods.

Class Welcome extends React. Component{
    // stateful component
    constructor(props){
        super(props);
        this.state = {name: 'Learning Hub'};
    }
    render(){
        return <h1>{this.state.name}</h1>;
    }
}

19. How To Use Styles in React?

In React, there are several ways to apply styles to components:

a) Inline styles:

const MyComponent = () => {
  const styles = {
    color: 'red,'
    fontSize: '16px',
  };

  return <div style={styles}>This text is styled</div>;
};

b) CSS Modules:

import React from 'react';
import styles from './MyComponent.module.css';

const MyComponent = () => {
  return <div className={styles.myClass}>This text is styled</div>;
};

c) CSS-in-JS libraries:

import React from 'react';
import styled from 'styled-components';

const StyledDiv = styled.div'
 color: blue;
  font-size: 18px;
';

const MyComponent = () => {
  return <StyledDiv>This text is styled</div>;
};

To learn more about CSS inline style, CSS Modules are used and how CSS and JavaScript work together to bring style to life. Follow this blog on CSS-in-JS and learn the benefits of CSS-in-JS over traditional CSS.

20. What Is the Difference Between HTML and React Event Handling?

The key differences between HTML and React event handling are:

• Naming convention:
• HTML: Event names are lowercase.

<button onclick="activateLasers()"></button>

• React: Event names use camelCase.

<button onClick={activateLasers}>

• Preventing default behavior:
• HTML: Return false to prevent default.

<a href="#" onclick="console.log('Clicked'); return false;">

• React: Call preventDefault() explicitly.

function handleClick(event) {
 event.preventDefault();
console.log('Clicked');
}

• Function invocation:
• HTML: Invoke function with parentheses.

<button onclick="activateLasers()">

• React: Pass function reference without parentheses.

<button onClick={activateLasers}>

These differences reflect React's more JavaScript-centric approach to handling DOM events, providing better integration with the component-based architecture and synthetic event system.

21. How To Use InnerHtml in React?

To use innerHTML-like functionality in React, you should use the dangerouslySetInnerHTML prop.

Here's how to use it:

• Create an object with a __html key containing your HTML string:

const htmlContent = {
  __html: '<h1>learning, <span style="color: red;">Hub</span></h1>'
};

• Use this object with the dangerouslySetInnerHTML prop on a React element:

function MyComponent() {
  return <div dangerouslySetInnerHTML={htmlContent} />;
}

Important points to remember:

• Security: Use dangerouslySetInnerHTML cautiously. Rendering untrusted content can expose your application to cross-site scripting (XSS) attacks.
• Performance: React can't optimize or update content inside dangerouslySetInnerHTML, so use it sparingly.
• Alternatives: Consider using React's built-in JSX syntax or creating custom components when possible.
• Naming: The "dangerous" in the prop name is intentional, serving as a reminder to be cautious.

22. How Are Events Different in React?

In React, events are handled differently from traditional DOM events. Here are the main differences:

• Naming Convention: React uses camelCase for naming events rather than lowercase.
• Passing Functions: Instead of passing a string representing the code to be executed, React events are passed a function that will be called when the event occurs.
• Event Pooling: React uses an "event pooling" technique to improve performance by reusing objects for event handlers, which means you cannot access the event object asynchronously.
• Synthetic Events: React events are not native browser events but are synthetic events wrapped around the native events to provide a more consistent interface across different browsers.
• Preventing Default Behavior: You need to call an event to prevent an event's default behavior.preventDefault() explicitly, as React doesn't automatically do this for you.

23. Why Does React Use className Over Class Attribute?

React's use of className instead of the class attribute is rooted in JavaScript language compatibility and the evolution of React itself. This choice reflects React's deep integration with JavaScript and its commitment to maintaining consistency across different versions and use cases.

The primary reasons for using className are:

• Compatibility and Conflict: Class is indeed a reserved keyword in JavaScript, which can lead to conflicts when used directly in JSX. This is why React uses className instead to manage CSS classes on elements.
• DOM API Alignment: React's decision to use className aligns with how class attributes are managed in the DOM API, ensuring consistency between React's virtual DOM and the browser's actual DOM manipulation methods.
• Historical Context: Before React 16, unknown attributes like class were ignored or caused warnings. With React 16 and later, these attributes are passed to the DOM but still generate warnings, emphasizing React's stance on attribute handling and compatibility.
• Consistency and Best Practices: Using className in JSX concerns compatibility and following best practices in React development. It ensures code consistency and reduces the risk of conflicts, aligning with React's design principles.

This change in attribute handling shows React's evolution in dealing with non-standard attributes. It also shows why using className remains the recommended approach, even though modern versions of React are more forgiving with attribute naming.

24. What Are Fragments?

Fragments can return multiple elements from a component's render method without creating an unnecessary wrapper DOM node. Fragments allow you to group a list of children without adding extra nodes to the DOM structure.

25. Why Are Fragments Better Than Container Divs?

Fragments in React offer several advantages over container divs:

• Fragments don't add extra nodes to the DOM, keeping it lighter and more efficient.
• With fewer DOM nodes, rendering and manipulation can be slightly faster.
• No need to style or target unnecessary div containers.
• Allows for more meaningful document structure without superfluous divs.
• Useful when mapping arrays to JSX, especially for table rows or list items.
• It helps avoid excessive depth in component hierarchies.

26. What Is the Use of the React-Dom Package?

Here are some of the main uses of the react-dom package you must know while preparing for React interview questions.

• Rendering Components: The ReactDOM.render() method renders a React component into a specific DOM element. It is done at the entry point of a React application, where the root component is rendered into the HTML document.
• Updating Components: The ReactDOM.render() method can also update an existing React component by rendering the new component into the same DOM element. React will efficiently update the DOM and reconcile changes between the previous and new component trees.
• Unmounting Components: To remove a React component from the DOM, the ReactDOM.unmountComponentAtNode() method is used, which unmounts it from the page.
• Finding DOM Elements: The ReactDOM.findDOMNode() method finds the actual DOM element associated with a React component instance. However, this method is deprecated in favor of using refs or callback functions to access DOM elements directly.
• Handling Events: The react-dom package provides a way to handle browser events in React components, such as onClick(), onSubmit(), etc.
• Testing Utilities: The react-dom/test-utils module offers utilities for testing React components. These utilities render components in a simulated DOM and simulate user interactions.

27. What Is the Difference Between React and ReactDOM?

Here are the differences between React and ReactDOM:

28. What Is ReactDOMServer?

The ReactDOMServer module allows React components to be rendered to static HTML markup on the server side, primarily used for Server-Side Rendering (SSR).

The ReactDOMServer object provides two main methods:

• renderToString(): This method renders a React component to its initial HTML string representation. The resulting string can then be sent to the client and hydrated (reused) on the client side to create the DOM tree.
• renderToStaticMarkup(): This method is similar to renderToString() but does not create any additional DOM attributes, such as data-reactroot, that React uses internally. It's useful for generating static HTML markup without any React-specific instructions.

29. What Are Higher-Order Components?

A higher-order component (HOC) is an advanced React technique for reusing component logic. Although HOCs are not explicitly part of the React API, they represent a pattern that arises from React's compositional nature.

30.How Do You Memorize a Component?

As an optimization technique, Memoization speeds up computer programs while consuming more memory space. The speed-up occurs because repetitive computations are avoided using cached results when the same input parameters are provided. However, the higher memory usage is due to the storage of these cached results.

React provides two built-in ways to memoize components:

• React.memo for functional components:
• Used for functional components.
• Wraps the component to prevent unnecessary re-renders.
• Performs a shallow comparison of props.

const MemoizedComponent = React.memo(function MyComponent(props) { // Component logic });

• React.PureComponent for class components:
• Used for class components.
• Automatically implements shouldComponentUpdate() with a shallow prop and state comparison.

class MyPureComponent extends React.PureComponent {
  render() {
    // Component logic
  }
}

31. How Do You Print JSON Pretty Well With React?

To print JSON pretty well in React, you can use the JSON.stringify() method with optional spaces or tabs to format the output.

Here's an example:

import React from 'react';

function PrettyPrintJSON({ data }) {
  const prettyJson = JSON.stringify(data, null, 2);

  return (
    <pre style={{
      backgroundColor: '#f4f4f4',
      padding: '10px',
      borderRadius: '5px',
      maxHeight: '400px',
      overflow: 'auto'
    }}>
      {prettyJson}
    </pre>
  );
}

// Usage example
function App() {
  const sampleData = {
    name: "JimJam",
    address: {
      street: "123 Main St",
      city: "Anytown",
      country: "USA"
    },
    hobbies: ["reading", "coding", "photography"]
  };

  return (
    <div>
      <h1>Pretty Printed JSON</h1>
      <PrettyPrintJSON data={sampleData} />
    </div>
  );
}

export default App;

32. How Is React Different From React Native?

Here's a comparison of React and React Native:

33. What Are the Recommended Ways To Check Static Type?

There are a few recommended ways for static type checking:

• PropTypes: PropTypes is a utility library that allows developers to define the expected data types for props passed to React components. These checks are performed only in development mode and serve as documentation for the expected data structure that a component needs.

Example:

MyComponent.propTypes = {
  message: PropTypes.string.isRequired
};

• TypeScript: TypeScript is a superset of JavaScript that adds optional static typing to the language. It's a powerful tool for static type checking in React applications, providing better tooling, improved code intelligence, and early error detection.

Example:

import React from 'react';

interface Props {
  message: string;
}

const MyComponent: React.FC<Props> = ({ message }) => {
  return <div>{message}</div>;
};

• Flow: Flow is a static type checker developed by Facebook, specifically for JavaScript. It adds static type annotations to your code, allowing you to catch potential issues during development. Flow is an alternative to TypeScript but has a smaller community and fewer third-party type definitions compared to TypeScript.

Example:

// @flow
import React from 'react';

type Props = {
  message: string
};

const MyComponent = (props: Props) => {
  return <div>{props.message}</div>;
};

34. What Are the Common Folder Structures for React?

There are several common folder structures used for organizing React projects. The choice of folder structure largely depends on the project's size, complexity, and team preferences. Here are some common folder structures:

• Level 1: Grouping by File Types
• Simple structure suitable for small to medium projects.
• Files are organized based on their type or function.

src/
├── components/
├── hooks/
├── services/
├── utils/
├── pages/
├── styles/
├── assets/
├── config/
├── App.js
└── index.js

• Level 2: Grouping by File Types and Features
• Suitable for medium to large projects.
• Combines file type organization with feature-based subfolders.

src/
├── components/
│   ├── common/
│   ├── auth/
│   ├── users/
│   └── products/
├── hooks/
│   ├── auth/
│   ├── users/
│   └── products/
├── services/
├── utils/
├── pages/
├── styles/
├── assets/
├── config/
├── App.js
└── index.js

• Level 3: Grouping by Features/Modules
• Ideal for large and complex projects.
• Highly modular approach with clear boundaries between different aspects of the application.

src/
├── modules/
│   ├── core/
│   │   ├── components/
│   │   ├── hooks/
│   │   ├── services/
│   │   └── utils/
│   ├── auth/
│   │   ├── components/
│   │   ├── hooks/
│   │   ├── services/
│   │   └── utils/
│   ├── users/
│   │   ├── components/
│   │   ├── hooks/
│   │   ├── services/
│   │   └── utils/
│   └── products/
│       ├── components/
│       ├── hooks/
│       ├── services/
│       └── utils/
├── shared/
│   ├── components/
│   ├── hooks/
│   └── utils/
├── assets/
├── config/
├── App.js
└── index.js

35. What Are the Popular Packages for Animation?

React provides a built-in solution called React.useState that allows you to build animation capabilities using JavaScript and user-defined hooks. However, for more advanced animation requirements, there are several popular animation libraries designed for React:

• React-Motion
• Animations
• React-transition-group.

36.Does the Lazy Function Support Named Exports?

Yes, the React.lazy function supports named exports. When using React.lazy with a named export module, you must provide an arrow function that returns the dynamic import for the named export.

37. How Do You Write Comments in React?

When it comes to React interview questions, you must know how to write comments in React. There are two types of comments used in React:

• Single-line comments: It has// and can describe a single line of code or temporarily disable a line of code. Here's an example:

// This is a single-line comment

• Multi-line comments: Multi-line comments are enclosed between /* and */. They can span multiple lines and are useful for providing more detailed explanations or temporarily commenting out large blocks of code. Here's an example:

/*
 * This is a multi-line comment
 * It can span multiple lines
 * and provide detailed explanations
 */

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Intermediate React Interview Questions and Answers

Now that we've covered the basics let's move on to more complex React interview questions.

38. What Are Hooks?

Hooks are functions that allow the use of state, and other React features in functional components. They were introduced in React 16.8 to enable developers to use stateful logic without writing a class component.

Some important points about hooks:

• They let you use state and lifecycle methods in functional components.
• Hooks always start with "use" (e.g., useState, useEffect).

Common built-in hooks include:

• useState: for adding state to functional components.
• useEffect: for performing side effects in components.
• useContext: for consuming context in a component.
• useRef: for creating mutable references.

Hooks must be called at the top level of your component, not inside loops, conditions, or nested functions. Custom hooks can be created to reuse stateful logic across components.

39. What Is the useState Hook in React?

The useState hook is a built-in React hook that allows adding a state to functional components. Before hooks, the state could only be used in class components. The useState hook returns an array with two elements:

• The current state value.
• A function to update the state value.

Syntax:

40. What Is the useEffect Hook in React?

The useEffect hook in ReactJS manages side effects like data fetching and DOM updates. Although it runs on every render, you can control its execution using a dependency array.

Its hook is similar to the lifecycle methods componentDidUpdate, componentDidMount, and componentWillUnmount in class components, but it combines their functionality into a single API.

Syntax:

41. What Is the Difference Between useState and useRef Hook?

Let's compare useState and useRef based on several aspects.

42. What Is a Wrapper Component?

A wrapper component is a component that wraps or contains another component. It can add additional functionality or props to the wrapped component.

43. What Are the Differences Between useEffect and useLayoutEffect Hooks?

44. What Is the Strict Mode in React?

The strict mode in React is a tool that helps identify potential problems in a React application by running extra checks and warnings. When strict mode is enabled, React will intentionally run certain functions twice to detect unexpected side effects and help developers identify them.

45. What Are the Benefits of Strict Mode?

Here are some benefits of using strict mode:

• Catches errors earlier: In strict mode, certain actions that would silently fail or have unexpected behavior in non-strict mode will throw errors. For example:
• Using undeclared variables
• Assigning to read-only properties
• Attempting to delete undeletable properties
• Using duplicate property names in object literals
• Prevents accidental global variables: In non-strict mode, if you forget to declare a variable with var, let, or const, it becomes a global variable. In strict mode, this throws a ReferenceError:

"use strict";
undeclaredVar = 10; // Throws ReferenceError

• Eliminates this coercion: In non-strict mode, when this is null or undefined in a function, it's automatically coerced to the global object. In strict mode, this remains as whatever it was set to:

"use strict";
function showThis() {
console.log(this);
}
showThis(); // logs undefined, not the global object

• Disallows duplicate parameter names: In strict mode, having multiple parameters with the same name in a function declaration throws a SyntaxError:

"use strict";
function duplicate(a, a) {} // SyntaxError

• Makes eval() safer: In strict mode, variables and functions declared inside an eval() statement are scoped to that eval:

"use strict";
eval("var x = 10");
console.log(x); // ReferenceError, x is not defined

• Forbids deleting plain names: In strict mode, you can't delete variables, functions, or function arguments:

"use strict";
var x = 10;
delete x; // SyntaxError

• Prohibits octal syntax: In strict mode, octal syntax (numbers starting with 0) is not allowed:

"use strict";
var octal = 010; // SyntaxError

• Disallows with statement: The with statement is not allowed in strict mode, as it can lead to confusion and performance issues:

 "use strict";
with (Math) {
x = cos(2); // SyntaxError
} 

• Throws error on invalid object literal property names: In strict mode, using invalid ECMAScript 5 property names in object literals throws an error:

 "use strict";
var o = { 019: "Invalid in ES5" }; // SyntaxError 

46. What Is the useReducer Hook? How the useReducer Hook Works

The useReducer hook is a React hook that manages complex state logic in functional components. It's particularly useful when you have state transitions that depend on the previous state or when the next state is determined by complex logic.

The useReducer hook works by implementing a state management pattern inspired by Redux. Here's a breakdown of how it functions:

• Initialization: When you call useReducer, you provide a reducer function and an initial state. React sets up the initial state and returns an array with two elements: the current state and a dispatch function.

• State Updates: You call the dispatch function with an action to update the state. An action is an object with a 'type' property describing the change to make.

• Reducer Execution: React invokes your reducer function when dispatch is called, passing it the current state and the action you dispatched.

• State Update: The reducer returns the new state. React then updates the state and re-renders the component with this new state.
• Performance Optimization: React uses Object.is() comparison to determine if the state has changed. If the new state is identical to the previous state, React skips re-rendering the component and its children.
• Closure Over Dispatch: The dispatch function doesn't change between re-renders, allowing you to pass it to child components without causing unnecessary re-renders.

This pattern allows for predictable state updates and centralized state management logic, which can be particularly useful in complex components or when state logic becomes intricate.

React Interview Questions Sheet

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47. How Does Context Work Using the useContext Hook?

The useContext hook in React allows one to subscribe to a React context and access its value without introducing nesting through render props or higher-order components. It provides a way to consume context data in functional components.

Here's how context works using the useContext hook:

• Creating a Context: First, you must use the React.createContext method. This returns an object with two properties: a Provider component and a Consumer component (or the useContext hook).

• Providing the Context Value: Within your component tree, wrap the components that need access to the context value with the Provider component and pass the value as a prop.

• Consuming the Context Value: In the components where you want to access the context value, you can use the useContext hook and pass the created context object as an argument.

When the component that uses the useContext hook is rendered, it will subscribe to the closest Provider component in the component tree that matches the context object passed to useContext. Whenever the Provider value updates, all components that have subscribed to that context using useContext will re-render with the new value.

Let’s see an example that demonstrates the usage of useContext:

48. What Is the React Memo Function??

Components in React are set to re-render when state or props values change. However, this can impact your application's performance because, even if the change only affects the parent component, every other child component attached to the parent component will re-render. When a parent component re-renders, so do all of its child components.

useCallback is a React Hook that helps to memoize function definitions between re-renders. This improves performance by ensuring that the function reference remains stable, which can prevent unnecessary re-renders of child components that receive these functions as props.

There are two common ways to use useCallback. The first is to define the callback directly within useCallback:

const memoizedCallback = useCallback(() => {
// function body
}, [/* dependencies */]);

Another option is to define the function separately and then wrap it with useCallback:

const someFunction = (arg) => {
// function body
};
const memoizedCallback = useCallback(someFunction, [/* dependencies */]);

In the example above, both approaches result in a memoized callback. Still, the second approach allows you to define the function body outside the useCallback call, which can be useful for more complex functions or when you want to reuse the function definition.

49. What Is the React Lazy Function?

The React.lazy function is a code-splitting technique that allows you to load components lazily. It is combined with dynamic import() statements to load components only when needed rather than upfront. This can help improve your application's initial load time by reducing the amount of code that needs to be parsed and evaluated initially.

Example of how to use React.lazy:

From the screenshot of the code above, React.lazy creates a lazy-loaded component called LazyComponent. The Suspense component renders a fallback component while the lazy component is loaded. When the lazy component is eventually loaded, it will replace the fallback component.

50. How Do You Pass an Event Handler to a Component?

You can pass event handlers and other functions as props to child components. The functions can be passed to the child component as below:

In the above example, LearningResource is a child component representing a learning resource in the Learning Hub. It receives resources and onStart props. The LearningHub component defines a handleStartReading function and passes it to each LearningResource. When clicking the StartReading button, this function updates the resource's state and displays an alert. Bypassing the function as a prop, you allow the child component to trigger actions in the parent component, updating the overall state of the learning hub.

51. What Is a Diffing Algorithm?

React uses a diffing algorithm to update the DOM efficiently in its reconciliation process. When a component's state changes, React creates a new virtual DOM and compares it with the current one. This comparison process, known as “diffing,” allows React to identify the minimal number of updates needed for the DOM.

52. What Are the Rules Covered by a Diffing Algorithm?

Here are some of the rules you must know while preparing for React interview questions:

• Elements of Different Types are Discarded and Re-created: If the root elements of the trees being compared have different types, React will tear down the old tree and build a new tree from scratch. For example, swapping a <div> with a <span> will cause a full re-render of the component and its children.
• Elements of the Same Type are Kept, with Their Attributes Updated: If the root elements of the trees being compared have the same type, React will update the attributes of the DOM node to match the new element but keep the underlying DOM node the same.
• Component Instances are Reused: When components are of the same type, React will update the existing instance with new props and state instead of creating a new component instance.
• Keys are Used to Identify Elements: When rendering lists of elements (e.g., using the map function), React relies on a unique key prop assigned to each element to identify which elements have been added, removed, or moved. React would have to re-render the entire list without keys for every update.
• Recursion on Children: The diffing algorithm recursively compares the components and elements within the component tree, applying the same rules at each level of the tree.

53. When Do You Need To Use Refs?

You need to use refs in React when you need to access the underlying DOM element or instance of a component. Some common use cases include:

• Managing focus, text selection, or media playback: Refs are useful when interacting with DOM elements not controlled by React, such as focusing an input field or playing/pausing a video.
• Imperative Animations: Refs can trigger imperative animations on DOM nodes.
• Integrating with Third-Party Libraries: When using third-party libraries that expect access to DOM nodes, you can use refs to provide that access.
• Accessing an Instance of a Component: In some cases, you may need to access an instance of a component, such as calling an imperative method on it.

54. Must the Prop Be Named Render for Render Props?

No, the prop doesn't have to be named render for render props. The rendered prop is just a convention and a pattern. You can use any name for the prop that serves the same purpose, like children or renderComponent.

55. Is It Recommended That the CSS-in-JS Technique Be Used in React?

Using CSS-in-JS in React is a popular approach and is generally recommended, especially for larger or more complex projects. CSS-in-JS provides several benefits:

• Scoped Styles: CSS-in-JS allows developers to create styles scoped to individual React components. This encapsulation minimizes naming conflicts and prevents unintended side effects, making style management more efficient in large codebases with numerous components.
• Dynamic and Conditional Styling: JavaScript's dynamic capabilities facilitate the creation of styles based on dynamic data or the state of components. CSS-in-JS libraries simplify the application of conditional styles, enabling responses to user interactions and adjustments to styles as application states change.
• Eliminating Global Scope Issues: Traditional CSS can introduce global scope issues and unintended style overrides. CSS-in-JS addresses this by encapsulating styles within components, reducing the risk of conflicts across different application areas.
• Theme and Variants Support: Many CSS-in-JS libraries come with built-in support for theming and style variants. This feature ensures consistent theming throughout the application and simplifies the creation of component variations with different styles.

56. Describe the Data Flow in React.

In React, data flows unidirectionally through props, from parent to child components. This is known as the top-down or unidirectional data flow.

• Parent to Child: Parent components pass data down to their child components via props. The child components receive these props as input and can render based on the received data.
• State Management: When dealing with data that needs to be updated based on user interactions or other events, React recommends managing the state in the nearest common ancestor component that needs to share that state with its descendants.
• Child-to-Parent Communication: Since data only flows from parent to child, child components cannot directly modify the state or props of their parent components. Instead, they use callback functions (passed down as props) to communicate changes back to the parent, which can then update its state and pass the new data back down via props.
• Context API: React provides the context API for sharing data across multiple levels of the component tree without passing props at every level. It allows data to be passed from a parent component to any nested child component without explicitly threading props through every level.
• State Management Libraries: In larger applications with more intricate state management, developers commonly use libraries like Redux or MobX. These third-party solutions offer a centralized store for state management and ensure unidirectional data flow.

57. Is It Better To Learn ES6 Before Starting With React?

Yes, it is highly recommended that you have a good understanding of ES6 (ECMAScript 2015) syntax and features before learning ReactJS. React heavily utilizes ES6 syntax and concepts, and many React examples and tutorials assume familiarity with ES6.

58. Explain Concurrent Rendering

Concurrent Rendering is a React feature that allows the framework to prepare multiple user interface versions simultaneously without blocking the main thread. It enables React to interrupt, pause, and resume rendering work, prioritizing more critical updates and maintaining UI responsiveness even during complex operations.

Key Features of Concurrent Rendering:

Non-blocking Updates:

• Renders UI updates in small, interruptible chunks.
• It keeps the main thread free for user interactions.

Prioritization:

• Assigns different priorities to various types of updates.
• Allows high-priority updates to interrupt lower-priority ones.

Time Slicing:

• It breaks rendering work into smaller time slices.
• Yields to the main thread between slices to handle other tasks.

Suspense Integration:

• Works with Suspense to handle asynchronous operations more gracefully.
• Improves management of loading states.

Interruptible Rendering:

• Can start and stop rendering as needed.
• Prevents display of outdated UI by abandoning obsolete renders.

Example: Let's consider a search interface in a large product catalog:

function ProductSearch() {
const [query, setQuery] = useState('');
const [results, setResults] = useState([]);

useEffect(() => {
// Simulating an API call
const fetchResults = async () => {
const data = await searchProducts(query);
setResults(data);
};

if (query) {
fetchResults();
}
}, [query]);

return (
<div>
<input 
value={query} 
onChange={(e) => setQuery(e.target.value)} 
placeholder="Search products"
/>
<Suspense fallback={<div>Loading...</div>}>
<ProductList results={results} />
</Suspense>
</div>
);
}

In this example, concurrent rendering allows React to:

• Keep the input responsive even while fetching and rendering results.
• Interrupt the rendering of outdated results if the user types quickly.
• Prioritize the display of new input over the rendering of search results.
• Use Suspense to manage the loading state of the ProductList component.

59. What Is the Difference Between Async Mode and Concurrent Mode?

Async Mode and Concurrent Mode refer to the same concept in React. The React team initially used the term "Async Mode" to describe this new rendering approach. However, they later renamed it "Concurrent Mode" to better reflect its capabilities and avoid potential confusion.

The terminology change from “Async Mode” to “Concurrent Mode” emphasized React's ability to work on updates with different priority levels and manage multiple rendering tasks simultaneously. This name more accurately captures the essence of the feature, which goes beyond just asynchronous operations.

60. What Is the Difference Between Imperative and Declarative in React?

Consider a simple UI component like a "Dark Mode" toggle switch. The app's theme switches between light and dark modes when you click it.

The imperative way of doing this might look like:

In this imperative approach, you directly manipulate the DOM, check the current state, and handle all necessary changes to update the UI. You have to manage adding and removing classes and updating the button text. It can become complex and error-prone in larger applications.

In contrast, the declarative approach in React might look like this:

In this declarative approach, you describe what the UI should look like based on the current state. React handles the DOM manipulation for you. You simply declare how the component should render in each state, and React efficiently updates the DOM when the state changes.

The declarative approach is easier to read, understand, and maintain. It separates the concerns of state management and UI rendering, making your code more predictable and less prone to bugs.

61. What Are the Benefits of Using TypeScript With ReactJS?

Using TypeScript with ReactJS provides several benefits:

• Type Safety: TypeScript adds static type checking to JavaScript, which can catch many potential bugs and type-related errors at compile-time before the code even runs. This helps prevent runtime errors and improves code quality.
• Better Developer Experience: TypeScript provides better tooling support, including autocompletion, code navigation, and refactoring capabilities, which can improve developer productivity and reduce the cognitive load when working with large codebases.
• Improved Code Documentation and Maintainability: TypeScript's type annotations serve as self-documenting code, making it easier to understand the expected data structures and function signatures. This can improve code maintainability, especially in larger projects or teamwork.
• Improved Scalability: As React applications grow in size and complexity, TypeScript's type system can help manage and scale the codebase more effectively, making it easier to refactor and add new features without introducing bugs.
• Better Tooling Integration: Many popular libraries and frameworks in the React ecosystem provide TypeScript support, enabling better integration and leveraging TypeScript's benefits across the entire project.
• Easier Refactoring: TypeScript's type-checking and tooling support makes it easier to refactor code safely, as the compiler can catch any breaking changes or type mismatches during the refactoring process.
• Improved Third-Party Library Adoption: TypeScript's type definitions make it easier to adopt and integrate third-party libraries into your React project, as the types provide a better understanding and documentation of the library's API.

62. How Is the New JSX Transform Different From the Old Transform?

The new JSX transform introduced in React 17 differs from the old transform in several ways:

• Automatic Runtime: In the new transform, the JSX runtime is automatically imported and injected by the compiler, eliminating the need for manual imports like import React from 'react' in every file that uses JSX.
• Improved Performance: The new transform generates more optimized and smaller code than the old one, resulting in better performance and reduced bundle sizes.
• No More React Import for JSX: With the new transform, you no longer need to import React in JSX files. The compiler handles the necessary imports automatically.
• Separation of Concerns: The new transform separates the concerns of rendering JSX elements and creating React elements, making the code more modular and easier to optimize.
• Support for Automatic Delegation: The new transform supports the automatic delegation of event handlers, reducing the need for manual event binding and improving performance.
• Better Developer Experience: The new transform provides better error messages and integration with tools like code editors and liters.
• Improved Tree Shaking: The new transform generates more tree-shakable code, allowing better elimination of unused code during the bundling process and further reducing bundle sizes.

63. What Are React Server Components?

React Server Components is a new feature introduced in React 18 that allows you to render React components on the server and send them to the client as part of the initial page load. This approach, known as Server-Side Rendering (SSR), can improve React applications' performance and perceived load times.

64. What Is Prop Drilling?

Prop drilling, or "threading," refers to passing data from a higher-level component down to a deeply nested child component through multiple levels of intermediate components. This is done by passing props from one component to the next, creating a "prop drilling" chain.

Here's an example of prop drilling:

// App.js
function App() {
const data = { message: 'Hello Reader!' };

return <ParentComponent data={data} />;
}

// ParentComponent.js
function ParentComponent(props) {
return <ChildComponent data={props.data} />;
}

// ChildComponent.js
function ChildComponent(props) {
return <GrandchildComponent data={props.data} />;
}

// GrandchildComponent.js
function GrandchildComponent(props) {
return <div>{props.data.message}</div>;
}

65. How Do You Use Callback and Memo Differ in React?

The main difference between useMemo and useCallback lies in their return types: useMemo returns a memoized value, whereas useCallback returns a memoized function.