Top 50+ React JS Interview Questions and Answers 2024

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Hello React enthusiasts! Have you ever felt nervous before a React JS interview? We get it! That's why we're thrilled to bring you the Top 50+ React JS Interview Questions and Answers. Whether you're a coding newbie or a seasoned developer, this guide is designed to make your React interview experience smoother than ever. No jargon, just straightforward answers to questions that matter.
Get ready to boost your confidence, impress interviewers, and take your React skills to new heights. Let's kick off this journey with Part 1 of our interview-friendly series!

React Interview Questions and Answers

Here's a list of top 50+ React interview questions along with their simple and easy-to-understand answers:

1. What is React?

Ans - React is an open-source JavaScript library developed by Facebook for building user interfaces or UI components. It is commonly used for building single-page applications where fast and interactive user experiences are essential.

2. What are the major features of React?

Ans - Major features of React include components for building UI, a virtual DOM for efficient updates, JSX for writing UI components in a syntax similar to HTML, unidirectional data flow, React Router for navigation, and the ability to manage state and props.

3. What is JSX?

Ans - JSX (JavaScript XML) is a syntax extension for JavaScript used with React. It allows developers to write HTML-like code within JavaScript, making it easier to describe the structure of UI components. JSX is later transpiled into regular JavaScript by tools like Babel.

4. What is the difference between Element and Component?

Ans - In React, an element is a plain JavaScript object that represents a DOM element or a component. An element is the smallest building block in React, while a component is a more advanced and reusable piece of code composed of one or more elements.

5. How to create components in React?

Ans - Components in React can be created using either class components or function components. Class components are ES6 classes that extend from `React.Component`, and function components are simple JavaScript functions. Both types can return JSX to describe what should be rendered.

6. When to use a Class Component over a Function Component?

Ans - Class components were traditionally used for more advanced features like state management and lifecycle methods. However, with the introduction of React Hooks, function components can now also manage state and use lifecycle methods, making them more popular. As of React 16.8, functional components with hooks are the recommended way to create components.

7. What are Pure Components?

Ans - Pure Components in React are a type of class component that automatically implements the `shouldComponentUpdate` method with a shallow prop and state comparison. This optimization can improve performance by preventing unnecessary renders when props or state haven't changed.

8. What is state in React?

Ans - State in React is a JavaScript object that represents the internal data of a component. It can be changed over time in response to user actions or other events. Components can have local state, managed by the component itself, and it is used for dynamic and interactive UIs.

9. What are props in React?

Ans - Props (short for properties) are a mechanism for passing data from a parent component to a child component in React. Props are immutable and help in making components more flexible and reusable by allowing them to receive data from outside sources.

10. What is the difference between state and props?

Ans - State: Represents the internal data of a component. It is mutable and can be changed by the component itself. Used for dynamic and interactive UIs.
Props: Short for properties, props are immutable and passed from a parent component to a child component. They are used to configure a component and make it customizable and reusable.

11. Why should we not update the state directly?

Ans - Directly updating the state in React is discouraged because React relies on the concept of virtual DOM and its ability to efficiently determine the difference between the current state and the new state to update the actual DOM. If you update the state directly, React might not be aware of the change, and it could lead to unexpected behavior. Instead, use the `setState` method, which ensures that React is notified of the state change and can perform the necessary updates.

12. What is the purpose of a callback function as an argument of `setState()`?

Ans - The callback function passed as an argument to `setState` is executed after the state is successfully updated. This is useful when you need to perform some action or execute code that relies on the updated state. It ensures that the code inside the callback runs after the state has been applied.

13. What is the difference between HTML and React event handling?

Ans - In React, event handling is similar to HTML, but there are some syntactical differences. In React, event names are camelCase (e.g., `onClick` instead of `onclick`). Additionally, in React, you pass a function as an event handler, whereas in HTML, you provide a string. React uses synthetic events to normalize the differences between browser implementations.

14. How to bind methods or event handlers in JSX callbacks?

Ans - There are a few ways to bind methods or event handlers in JSX callbacks:
- Use arrow functions in the callback directly: `onClick={() => this.handleClick()}`
- Bind the method in the constructor: `constructor() { this.handleClick = this.handleClick.bind(this); }`
- Use class properties syntax: `handleClick = () => { /* method code */ }`

15. How to pass a parameter to an event handler or callback?

Ans - To pass parameters to an event handler in React, you can use arrow functions or the `bind` method. For example:
- Arrow function: `onClick={() => this.handleClick(param)}`
- Binding in the constructor: `onClick={this.handleClick.bind(this, param)}`

16. What are synthetic events in React?

Ans - Synthetic events in React are wrappers around the native browser events, providing a consistent interface across different browsers. React creates synthetic events to ensure that the event handling behavior is the same across various environments. This abstraction helps in normalizing the differences in event handling between browsers.

17. What are inline conditional expressions?

Ans - Inline conditional expressions in React allow you to conditionally render content based on a condition. You can use the ternary operator or logical `&&` to achieve this. For example:
```jsx
{condition ? <TrueComponent /> : <FalseComponent />}
```

18. What is the "key" prop, and what is the benefit of using it in arrays of elements?

Ans - The "key" prop is a special attribute used in React when rendering arrays of elements. It helps React identify which items have changed, been added, or been removed. Using keys improves the efficiency of rendering and updating components, especially in the context of dynamic lists. Keys should be unique among siblings, but they don't need to be globally unique.

19. What is the use of refs?

Ans - Refs in React provide a way to access and interact with the DOM directly or to get a reference to a React component. They are useful in scenarios like accessing input values, triggering imperative animations, or integrating with third-party DOM libraries that don't work well with React's declarative approach.

20. How to create refs?

Ans - Refs can be created using the `React.createRef()` method. For example:
```jsx
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.myRef = React.createRef();
}
render() {
return <div ref={this.myRef}>Hello, World!</div>;
}
}
```
After creating a ref, you can access the DOM node or React component it refers to using `this.myRef.current`.

21. What are forward refs?

Ans - Forwarding refs is a technique in React that allows a component to pass its `ref` property to a child component. This enables the parent component to interact with the child's DOM node or React component. The `React.forwardRef` function is typically used to create components that forward refs.

22. Which is the preferred option within callback refs and `findDOMNode()`?

Ans - Using callback refs is generally preferred over `findDOMNode()`. Callback refs provide a more flexible and explicit way to get a reference to a DOM node or React component. `findDOMNode()` is considered legacy, and using it can make it harder to follow the data flow in your application. Callback refs are more in line with React's modern ref handling.

23. Why are String Refs legacy?

Ans - String refs (using `ref="myRef"`) were the original way to create refs in React. However, they are considered legacy, and the use of callback refs (functions) or `React.createRef()` is now recommended. String refs have some drawbacks, such as not being a reliable way to access refs in function components and being prone to namespace collisions.

24. What is Virtual DOM?

Ans - The Virtual DOM (Document Object Model) is a programming concept used in React to improve the performance of updating the user interface. It is a lightweight copy of the actual DOM, maintained by React. When the state of a component changes, React first updates the Virtual DOM, compares it with the previous state, and then efficiently updates only the parts of the real DOM that have changed.

25. How does the Virtual DOM work?

Ans - When a component's state changes in React, a new Virtual DOM tree is created. React then compares this new tree with the previous one to identify the differences (diffing). Once the differences are identified, React calculates the most efficient way to update the real DOM to reflect these changes. This process of updating the real DOM is called reconciliation.

26. What is the difference between Shadow DOM and Virtual DOM?

Ans - Virtual DOM: A concept used in React to optimize the updating of the actual DOM by first updating a lightweight copy (virtual DOM) and then efficiently applying the changes to the real DOM.

Shadow DOM: A web standard that allows encapsulation of styling and structure in a web component, providing a way to create self-contained components with their own encapsulated DOM.

27. What is React Fiber?

Ans - React Fiber is a complete rewrite of the core algorithm that React uses to reconcile the Virtual DOM and update the UI. It was introduced to improve the performance and enable new features like asynchronous rendering and better support for concurrent updates.

28. What is the main goal of React Fiber?

Ans - The main goal of React Fiber is to improve the responsiveness and performance of React applications by enabling asynchronous rendering and better handling of concurrent updates. It allows React to break down the work into smaller units and prioritize rendering based on the priority of each unit.

29. What are controlled components?

Ans - Controlled components in React are components whose state is controlled by React. The state of a controlled component is typically handled by the parent component, and the child component receives its state and updates through props. This ensures that the parent has full control over the child component's behavior.

30. What are uncontrolled components?

Ans - Uncontrolled components in React are components that manage their own state internally. Instead of being controlled by the parent component through props, an uncontrolled component directly manages its own state. This is often done using refs to access and modify the DOM directly. Uncontrolled components are less common than controlled components in React applications.

31. What is the difference between `createElement` and `cloneElement`?

Ans - `createElement`: It is a method used to create and return a new React element. It takes three arguments: the type of the element (string or React component), an optional set of properties (props) for the element, and the element's children.
`cloneElement`: It is a method used to clone and return a React element with a new set of properties. It takes an element to clone as the first argument and an optional set of new properties as the second argument. It is often used to extend or override the properties of an existing element.

32. What is Lifting State Up in React?

Ans - Lifting State Up is a pattern in React where the state is moved from a child component to its parent component. This is done to share state between components that need access to the same data. By lifting the state up to a common ancestor, you avoid prop-drilling (passing props through multiple layers of components) and make the state more accessible to the components that need it.

33. What are the different phases of component lifecycle?

Ans - The component lifecycle in React has three main phases:
1. Mounting: The phase where a component is being created and inserted into the DOM.
2. Updating: The phase where a component is being re-rendered as a result of changes in either its props or state.
3. Unmounting: The phase where a component is being removed from the DOM.

34. What are the lifecycle methods of React?

Ans - Mounting Phase:
- `constructor()`
- `static getDerivedStateFromProps()`
- `render()`
- `componentDidMount()`
Updating Phase:
- `static getDerivedStateFromProps()`
- `shouldComponentUpdate()`
- `render()`
- `getSnapshotBeforeUpdate()`
- `componentDidUpdate()`
Unmounting Phase:
- `componentWillUnmount()`

35. What are Higher-Order components?

Ans - Higher-Order Components (HOCs) are functions that take a component and return a new component with enhanced functionality. HOCs allow you to reuse component logic, share code between components, and modify the behavior of components. They are a powerful pattern in React for code reuse.

36. How to create a props proxy for an HOC component?

Ans - To create a props proxy for an HOC component, you typically pass the original component as an argument to the HOC function and return a new component. Here's a simplified example:
```jsx
const myHigherOrderComponent = (WrappedComponent) => {
return class extends React.Component {
render() {
return <WrappedComponent {...this.props} extraProp="additional data" />;
}
};
};
```

37. What is context?

Ans - Context in React is a way to pass data through the component tree without having to pass props manually at every level. It is often used when data needs to be accessible to many components at different levels of the component tree.

38. What is the children prop?

Ans - The `children` prop is a special prop in React that represents the content between the opening and closing tags of a component. It is used to pass elements, components, or text content as children to a component. You can access and manipulate the `children` prop within the component to modify or render content.

39. How to write comments in React?

Ans - In JSX, comments are written inside curly braces `{/* */}`. For example:
```jsx
<div>
{/* This is a comment */}
<p>Hello, World!</p>
</div>
```

40. What is the purpose of using the super constructor with a props argument?

Ans - When you define a constructor in a React component that extends `React.Component`, you need to call `super(props)` within the constructor. This is necessary to ensure that the component class properly inherits from `React.Component` and sets up the initial state and other internal mechanisms. It is a requirement in ES6 classes when defining a constructor in a class that extends another class.

41. What is reconciliation?

Ans - Reconciliation is the process by which React updates the DOM to match the virtual DOM after a component's state or props change. React's diffing algorithm identifies the differences between the previous and current state or props, and then efficiently updates only the parts of the DOM that have changed.

42. How to set state with a dynamic key name?

Ans - You can set state with a dynamic key name using the computed property name syntax introduced in ECMAScript 2015 (ES6). Here's an example:
```jsx
handleInputChange = (key, value) => {
this.setState({ [key]: value });
};
// Usage
this.handleInputChange("dynamicKey", "dynamicValue");
```

43. What would be the common mistake of a function being called every time the component renders?

Ans - One common mistake is not memoizing or properly handling functions passed as props. If a new function instance is created on every render, it can cause unnecessary re-renders of child components, affecting performance. To avoid this, use callback functions or memoization techniques.

44. Is the lazy function supports named exports?

Ans - As of my last knowledge update in January 2022, the `lazy` function in React primarily supports default exports. However, support for named exports was an open issue in the React repository. Please check the latest React documentation or release notes for any updates or changes.

45. Why does React use `className` over the `class` attribute?

Ans - React uses `className` instead of the traditional `class` attribute to define HTML class names. This is because `class` is a reserved keyword in JavaScript, and using it in JSX would lead to a syntax error. Therefore, React uses `className` to set the CSS class of an element.

46. What are fragments?

Ans - Fragments are a feature in React that allows you to group multiple children elements without introducing an additional parent wrapper element in the DOM. Fragments do not create extra nodes in the DOM hierarchy. They provide a cleaner way to structure your JSX when you need to return multiple elements without a common parent.

47. Why are fragments better than container divs?

Ans - Fragments are preferred over container divs when you want to group multiple elements without introducing an extra DOM node. Using container divs can lead to unnecessary and sometimes semantically incorrect markup. Fragments avoid this issue by allowing you to group elements without adding any extra nodes to the DOM.

48. What are portals in React?

Ans - Portals in React provide a way to render children components outside their parent hierarchy, typically at the top level of the DOM or in a different container. This can be useful for rendering components like modals that need to appear above all other elements but remain logically related to the rest of the application.

49. What are stateless components?

Ans - Stateless components, also known as functional components, are components in React that are defined as plain JavaScript functions. They do not have internal state (using the `useState` hook) and do not have lifecycle methods. Stateless components are mainly used for presenting UI based on the input props they receive.

50. What are stateful components?

Ans - Stateful components, also known as class components or components using hooks, are components in React that can have and manage their own internal state using the `state` property or `useState` hook. Stateful components can also implement lifecycle methods and handle user interactions, making them suitable for managing dynamic and interactive parts of the UI.

51. How to apply validation on props in React?

Ans - You can apply validation on props in React by using PropTypes. PropTypes is a type-checking library that helps you define the types of props a component should receive. Here's an example:
```jsx
import PropTypes from 'prop-types';

MyComponent.propTypes = {
name: PropTypes.string.isRequired,
age: PropTypes.number,
isStudent: PropTypes.bool.isRequired,
};
```

52. What are the advantages of React?

Ans - Declarative Syntax: React uses a declarative syntax that makes it easier to understand and reason about the code.
Component-Based: The component-based architecture promotes reusability and maintainability.
Virtual DOM: React's virtual DOM enhances performance by efficiently updating the actual DOM.
-One-Way Data Binding: React follows a unidirectional data flow, making it easier to manage and understand state changes.
Ecosystem: React has a vast ecosystem with a strong community, supporting libraries, and tools.

53. What are the limitations of React?

Ans - Steep Learning Curve: React can have a steep learning curve, especially for beginners.
JSX Complexity: JSX syntax might be challenging for developers coming from HTML backgrounds.
Overhead of Virtual DOM: While the virtual DOM improves performance, it introduces a certain amount of overhead.
Tooling: The tooling around React can be overwhelming, and choosing the right set of tools can be challenging.

54. What are error boundaries in React v16?

Ans - Error boundaries are a feature in React v16 and later that allow components to catch JavaScript errors anywhere in their component tree, log those errors, and display a fallback UI instead of crashing the entire application. To use error boundaries, you define a special method called `componentDidCatch` in your component.

55. How are error boundaries handled in React v15?

Ans - In React v15 and earlier, error boundaries were not supported. Errors in components could lead to the entire application crashing. React v16 introduced error boundaries to address this issue and provide a way to gracefully handle errors within components.

56. What are the recommended ways for static type checking?

Ans - For static type checking in React, you can use TypeScript or Flow. TypeScript is a superset of JavaScript that adds static typing, and Flow is a static type checker developed by Facebook. Both tools help catch type-related errors during development and provide better tooling support.

57. What is the use of the react-dom package?

Ans - The `react-dom` package is responsible for rendering React components in the DOM (Document Object Model). It provides methods like `render` for mounting React components into the browser, `hydrate` for server-side rendering, and other utilities for interacting with the DOM.

58. What is the purpose of the render method of react-dom?

Ans - The `render` method in `react-dom` is used to render a React element into the DOM in a specified container. It takes two arguments: the element to render and the container DOM element where the rendering should take place. For example:
```jsx
import React from 'react';
import ReactDOM from 'react-dom';

const element = <h1>Hello, World!</h1>;
ReactDOM.render(element, document.getElementById('root'));
```

59. What is ReactDOMServer?

Ans - `ReactDOMServer` is a package in React that provides server-side rendering APIs. It allows you to render React components to static HTML on the server, making it possible to send pre-rendered HTML to the client, which can improve initial page load performance.

60. How to use InnerHTML in React?

Ans - In React, the use of `innerHTML` is discouraged due to potential security vulnerabilities (e.g., Cross-Site Scripting). If you need to render HTML content, you can use the `dangerouslySetInnerHTML` prop. However, it should be used with caution, and you must ensure that the HTML content is sanitized to prevent security issues.

Also Read: 50+ Advanced React Interview Questions and Answers 2024

--Thank You--