Top 50+ React Interview Question & Answers

React Interview Questions and Answers for Beginners

1. Describe What is React.

Facebook created the front-end JavaScript library React in 2011. It uses a component-based methodology that facilitates the creation of reusable user interface components. It's used for the development of intricate and dynamic mobile and online user interfaces.

2. What characteristics does React have?

React's key characteristics are as follows:

• It supports Server-side rendering.
• Since real DOM actions are costly, it will use the virtual DOM (Data Object Model) instead of the real one.
• It adheres to data flow or unidirectional data binding.
• To construct the view, it makes use of reusable or composable user interface components.

Read More - React Developer Salary in India

3. What are the most important benefits of utilizing React?

Several significant benefits of React include:

• Learning and using React is simple.
• React adheres to the MVC design pattern.
• React increases efficiency by utilizing Virtual DOM.
• It's simple to create dynamic web applications.
• React is conducive to SEO.
• Reusable components are allowed by React.
• In favor of practical tools.
• React comes with a large library.
• Range of code testing.

4. What are the main drawbacks of React?

The following is a list of React's main drawbacks:

• It's only a library, React. It is not a comprehensive structure.
• Its extensive library requires time to fully comprehend.
• The novice programmers could find it challenging to comprehend and write code.
• React makes use of JSX and inline templating, which can be challenging and problematic. It complicates the coding as well.

5. Describe JSX.

For JavaScript XML, use JSX. It is a React extension that makes it possible to write JavaScript code with an HTML-like appearance. It facilitates the understanding of HTML files. The JSX file strengthens and improves the functionality of the React application. In the same file where you write JavaScript code, JSX enables you to write XML-like syntax. Preprocessors (i.e., transpilers like Babel) subsequently convert these expressions into JavaScript code.

6. Why is JSX unreadable by browsers?

Only JavaScript objects can be read by browsers, but JSX isn't just any old JavaScript object. Therefore, to allow a browser to read JSX, we must first use JSX transformers like Babel to convert the JSX file into a JavaScript object, which we can then send to the browser.

7. Why is JSX used by us?

• Because optimization is done as the code is being translated to JavaScript, it is faster than standard JavaScript.
• React uses components that combine both markup and logic, as opposed to dividing the two technologies into different files.
• The majority of the problems can be discovered during compilation, and it is type-safe.
• It facilitates the creation of templates.

8. What does Virtual DOM mean to you?

Initially, a virtual DOM is simply a duplicate of the actual DOM, created with lightweight JavaScript objects. The elements, their characteristics, and the content as Objects and their properties are listed in a node tree. React's render function takes its component parts and turns them into a node tree. After that, it modifies this tree in response to changes made to the data model by the system or by the user through a variety of activities.

9. Describe how Virtual DOM operates.

Virtual DOM operates in three stages:

1. The complete user interface is re-rendered in a virtual DOM representation whenever any data changes in the React application.
2. Next, the difference between the new DOM and the old DOM representation is evaluated.
3. After the computations are finished, only the modified items are updated in the real DOM.

Read More - React Roadmap

10. What does React's render() method achieve?

Every react component needs to have a render() function. The HTML that should be displayed in the component is returned by this function. Every element that needs to be rendered must be contained inside a single parent tag, such as <div> or <form>.

11. How may multiple components be integrated into one?

The following React methods allow you to combine two or more components into one:

• JSX Composition
• Passing Components as Props
• Conditional Rendering
• Higher-Order Components (HOCs)
• Render Props

12. What Do React Props Mean?

Properties are shortened to props in react. It is a built-in React object that functions similarly to HTML attributes and stores the value of a tag's attributes. Data transfer between components is made possible with props. Similar to how arguments are supplied to a function, props are also passed to the component.

13. In React, what is a State?

React components consist of states at their core. Data originates from states, which should be kept as basic as feasible. In essence, states in react are the objects that control how components are rendered and behave. Unlike the props, they can be changed to create dynamic and interactive elements. They can be accessible through this.state().

14. Distinguish in React between States and Props.

The following lists the main distinctions between States and Props.

15. In React, how can the State of a component be updated?

Using this.setState() function, we can modify a component's State. The State is not always instantly replaced by this technique.

Example:

import React, { Component } from 'react';
class Counter extends Component {
 constructor(props) {
 super(props);
 this.state = {
 count: 0,
 };
 }
 incrementCount = () => {
 this.setState({ count: this.state.count + 1 });
 };
 render() {
 return (
 <div>
 <h1>Counter</h1>
 <p>Count: {this.state.count}</p>
 <button onClick={this.incrementCount}>Increment</button>
 </div>
 );
 }
}
export default Counter;    

React Interview Questions and Answers for Intermediate

16. Define the differences between React's stateless and stateful components.

17. What is the purpose of an arrow function in React?

Arrow functions are essentially a condensed form of function expression writing syntax. The functions are also referred to as "fat arrows" (=>). Due to ES6's default lack of auto-binding, these functions enable the components' contexts in react to be bound correctly. Most of the time, arrow functions come in handy when interacting with higher-level functions.

18. What does React define as an event?

Events in React are the responses that are set off by particular actions, such as key presses, mouse clicks, and mouse hovers. Managing these events is comparable to managing DOM element events. However, there are some syntactical variations, such as the following:

1. Events are named in camel case rather than only lowercase.
2. Functions are passed in place of strings when passing events.

19. How may an event be created in React?

You may create a React event by carrying out the following example:

import React, { Component } from 'react';
class MyComponent extends Component {
 // Event handler function
 handleClick = () => {
 alert('Button clicked!');
 }
 render() {
 return (
 <div>
 <h1>React Event Example</h1>
 <button onClick={this.handleClick}>Click me</button>
 </div>
 );
 }
}
export default MyComponent;    

20. What do React's synthetic events mean?

• The objects that serve as a cross-browser wrapper for the native event of the browser are known as synthetic events.
• They create a single API that combines several browsers' behaviors.
• This is done to ensure that the properties displayed by the events are consistent across various browsers.

21. How do controlled and uncontrolled components in React vary from one another?

22. Describe Lists in React.

• Data can be shown in an ordered way using lists. Lists in React can be formed similarly to how they are in JavaScript.
• With the help of the map() method, we may go through the list's elements.

23. What use do keys provide in React?

• The UI is driven by data that corresponds to distinct Virtual DOM Elements, which are identified by keys.
• By reusing every element already present in the DOM, they assist React in rendering as efficiently as possible.
• These keys have to be distinct strings or numbers, or else React simply rearranges the elements rather than re-rendering them.
• The performance of the application increases as a result.

24. How are React forms created?

• HTML forms and React forms are comparable.
• However, with React, the state is updated exclusively through setState() and is stored in the component's state property.
• As a result, a JavaScript function manages the elements' submissions and prevents them from immediately updating their state.
• The information that a user enters into a form is fully accessible to this function.

25. What stages does the lifecycle of a React component go through?

The life cycle of a React component has the following phases:

1. Initial Phase
2. Mounting Phase
3. Updating Phase
4. Unmounting Phase

26. Define the lifecycle methods of React components.

The important React lifecycle methods are:

• getInitialState()
• componentWillMount()
• componentDidMount()
• componentWillReceiveProps()
• shouldComponentUpdate()
• componentWillUpdate()
• componentDidUpdate()
• componentWillUnmount()

27. What do React's Pure Components mean?

The simplest and fastest components that can be written are called pure components. Any component with merely a render() can be swapped out. These elements improve the application's functionality and code simplicity.

28. What are React's Higher Order Components (HOC)?

• React's higher-order components are a sophisticated React method for reusing component logic.
• This function accepts a component as input and outputs a new one. Stated otherwise, this function takes another function as an input.
• The official website states that it is a pattern that results from React's compositional structure rather than a feature or part of the React API.

29. How does React's HOC work?

HOC can be used for a wide range of tasks, some of which are listed below:

• Code Reusability
• Props manipulation
• State manipulation
• Render highjacking

30. What distinguishes a React component from an element?

Elements and components in react differ mostly in the following ways:

React Interview Questions for Experienced

31. What is the React commenting process?

We can write comments in React just like we would in JavaScript. Two possible methods are:

1. Comments in a single line
2. Multiple-line comments

32. Why does React need component names to begin with a capital letter?

The component names in React must begin with a capital letter. The component name will throw an error as an unrecognized tag if we start it in lowercase. The reason behind this is that JSX treats tag names in lowercase HTML tags.

33. In React, what are fragments?

React components that return multiple elements employ fragments. It lets you group a list of several offspring without expanding the DOM with another node.

34. Why do fragments work better in React than container divs?

• Because fragments do not generate additional DOM nodes, they operate more quickly and use less memory.
• Maintaining the intended layout might be challenging with certain CSS styling, such as CSS Grid and Flexbox, which introduce <div> tags in the middle and have unique parent-child relationships.
• There is less clutter in the DOM Inspector.

35. How can I use React validation on props?

• Props validation is a tool that aids developers in preventing faults and issues in the future.
• It improves the readability of your code.
• PropTypes is a unique property used by React components that helps you find errors by verifying the data types of values supplied through props.
• You do not have to define components with propTypes in order to use them.
• We may use App.propTypes in the React component to apply validation on props.
• The JavaScript console will display warnings if any properties are supplied with an invalid type.
• You must set the App.defaultProps after defining the validation rules.

36. Describe create-react-app.

Facebook announced the Create React App tool to help developers create React applications. You can use it to construct React apps that are one page in length. You can avoid time-consuming setup and configuration tasks like Webpack or Babel by using the preset create-react app.

37. In React, how do you make a component?

In React, there are two ways to construct a component:

• Function Components: The easiest method for creating a component in React is this one. These are the only JavaScript functions that return React elements and take an object called props as their first parameter.
• Class Components: You can define a component using the ES6 class and the class components function makes it easier.

38. What exactly are React hooks?

React state and lifecycle aspects can be "hooked into" from a functional component using hooks, which are functions. Class components are not compatible with React Hooks. They permitted us to compose parts without a class. Common hooks include useState, useEffect, and useContext.

39. Which guidelines should you adhere to when using React's hooks?

When using Hooks in your code, you have to abide by two rules:

1. React Hooks can only be invoked at the highest level. Calling them from within loops, conditions, or nested functions is prohibited.
2. Calling the Hooks is only permitted from React Function Components.

40. How do forms work in React?

• Forms are used by React to facilitate user interaction with web applications.
• Users can interact with the program and enter the necessary data whenever needed by using forms.
• Certain elements are present in forms, including radio buttons, checkboxes, buttons, and text fields.
• Numerous functions, including user identification, searching, filtering, indexing, and more, are accomplished by using forms.

41. How do you interpret the term "props" in React?

Props are inputs to components in React. They have naming conventions similar to HTML tag attributes and can be single values or objects holding a set of values supplied to components upon creation. They are information that a parent component transmits to a child component. The following component functionality is the primary function of props in React:

• Give your component access to custom data.
• The status of the trigger changes.
• Use via this.props.reactProp within component's render() method.

42. How do refs work in React?

• The abbreviation for References in React is Refs.
• This attribute serves to keep a reference to a specific React element or component that the render configuration function of the component will return.
• It is employed to provide pointers to specific elements or components that render() has returned.
• They are useful when we need to add methods to the components or when we need to measure the DOM.

43. What are React's Forward Refs and when are they used?

• One feature that is utilized to send a reference from one component to one of its child components is called ref forwarding.
• The React.forwardRef() function can be used to carry it out.
• It works very well with higher-order components and is utilized specifically in libraries of reusable components.

44. In React, which is the better choice—callback references or findDOMNode()?

• Callback references are a better solution than the findDOMNode() API.
• Because findDOMNode() precludes some future enhancements in React, and callback refs provide better control when the refs are set and unset.

45. What is React Router and how does React require it?

• Developed on top of React, React Router is a robust routing library that facilitates the addition of new screens and processes to the application.
• By doing this, the URL and the data that is shown on the webpage are kept up to date.
• It is used to create single-page web apps and has a consistent structure and behavior.
• It's easy to use React Router's API.

46. Describe React Router's benefits.

The following list includes React Router's key benefits:

• It is not required to manually set the browser history in this case.
• Link is used to navigate the application's internal links. It is comparable to the anchor element.
• For rendering, the Switch function is used.
• There is only one Child element required by the router.
• All of the components are listed in <Route> in this.
• The three packages that make up the packages are Web, Native, and Core. It supports the React application's small size.

47. What distinguishes Conventional Routing from React Routing?

React routing and conventional routing differ in the following ways:

48. Describe the concept of flux.

The unidirectional data flow is enforced by the architectural pattern known as flux. Utilizing a central store with authority over all data, it facilitates communication between various components and manages generated data. Any updates to the data across the application must happen exclusively here. Flux lowers run-time errors and gives the program stability.

49. Explain Redux.

One of the most popular front-end development libraries available today is Redux. It is utilized for the overall state management of JavaScript applications and provides a predictable state container. Redux-developed applications are simple to test and exhibit consistent behavior across many settings.

50. What three key principles does Redux adhere to?

• One and only source of truth: An object/state tree inside a single store contains the state of the entire application. It is simpler to debug or inspect the program and to keep track of changes over time using a single state tree.
• Read-only status: An action must be triggered in order to alter the state. A simple JS object that describes the change is called an action. The action is the minimal representation of a change to the data, the same as the state is the minimal representation of data.
• Modifications are made using pure functions: Pure functions are required to define the actions' transformation of the state tree. Functions classified as pure have a return value that is exclusively determined by the values of their inputs.

51. What is the difference between useEffect and componentDidMount?

52. How can you optimize performance in a React application?

• Using React's PureComponent or React.memo to avoid unnecessary re-renders.
• Implementing lazy loading with React.lazy and Suspense for code splitting.
• Using the useCallback and useMemo hooks to memoize functions and values.
• Avoiding inline functions and objects as props.
• Minimizing the use of the global state and keeping components as stateless as possible.

53. Explain the Context API in React.

• It provides a way to share values between components without passing props manually at every level.
• It is used to manage the global state in a React application.
• The createContext function creates a context, and Provider and Consumer components are used to pass and consume the context value, respectively.

54. What is React Suspense?

• React Suspense is a feature that allows components to wait for something like data fetching before rendering.
• It provides a way to declare a loading state while the component waits for asynchronous operations to complete.
• It works well with React.lazy for lazy loading components.

55. How does the useReducer hook work?

• The useReducer hook is used for managing complex state logic in React.
• It accepts a reducer function and an initial state and returns the current state and a dispatch function.
• It is similar to Redux but is built into React.
• The reducer function specifies how the state should change in response to actions dispatched by the component.

56. How do you handle forms in React?

• Forms in React can be handled using controlled or uncontrolled components.
• In controlled components, form data is handled by the component's state, typically using the useState hook.
• In uncontrolled components, data is handled by the DOM itself, usually with refs to access form values.

57. What are React Portals and how do you use them?

• React Portals provide a way to render children into a DOM node that exists outside the hierarchy of the parent component.
• They are used for rendering elements like modals, tooltips, or dropdowns that need to visually break out of the parent container.
• You can create a portal using ReactDOM.createPortal(child, containerNode).

58. How do you handle errors in React components?

• Errors in React components can be handled using error boundaries.
• An error boundary is a component that catches JavaScript errors in its child component tree, logs those errors, and displays a fallback UI.
• It is created using a class component with the componentDidCatch lifecycle method and a getDerivedStateFromError static method.

59. What is React.memo and when would you use it?

• React.memo is a higher-order component that prevents a functional component from re-rendering if its props haven't changed.
• It is used for performance optimization by memoizing the component and is useful when you have a component that renders the same output given the same props.

60. Explain the difference between useRef and createRef.

• useRef is a hook that returns a mutable ref object whose .current property is initialized to the passed argument.
• It persists between renders and can be used to store a reference to a DOM element or any mutable value. createRef is a method used in class components to create a ref.
• It returns a new ref object that can be attached to a DOM element via the ref attribute.
• useRef is typically used in functional components, while createRef is used in class components.

Summary