Top 10 React js interview questions.
Tags: react, interview, javascript, nextjs
Published: 2023-01-24 04:58:22


As a React developer, it is important to have a solid understanding of the framework's key concepts and principles. With this in mind, I have put together a list of 10 important questions that every React developer should know, whether they are interviewing for a job or just looking to improve their skills.

Before diving into the questions and answers, I suggest trying to answer each question on your own before looking at the answers provided. This will help you gauge your current level of understanding and identify areas that may need further improvement.

Let's get started!


## 01. What is React and what are its benefits?
**Ans:** React is a JavaScript library for building user interfaces. It is used for building web applications because it allows developers to create reusable UI components and manage the state of the application in an efficient and organized way.

## 02. What is the virtual DOM and how does it work?
**Ans:** The Virtual DOM (Document Object Model) is a representation of the actual DOM in the browser. It enables React to update only the specific parts of a web page that need to change, instead of rewriting the entire page, leading to increased performance.

When a component's state or props change, React will first create a new version of the Virtual DOM that reflects the updated state or props. It then compares this new version with the previous version to determine what has changed.

Once the changes have been identified, React will then update the actual DOM with the minimum number of operations necessary to bring it in line with the new version of the Virtual DOM. This process is known as "reconciliation".

The use of a Virtual DOM allows for more efficient updates because it reduces the amount of direct manipulation of the actual DOM, which can be a slow and resource-intensive process. By only updating the parts that have actually changed, React can improve the performance of an application, especially on slow devices or when dealing with large amounts of data.

## 03. How does React handle updates and rendering?
**Ans:** React handles updates and rendering through a virtual DOM and component-based architecture. When a component's state or props change, React creates a new version of the virtual DOM that reflects the updated state or props, then compares it with the previous version to determine what has changed. React updates the actual DOM with the minimum number of operations necessary to bring it in line with the new version of the virtual DOM, a process called "reconciliation". React also uses a component-based architecture where each component has its own state and render method. It re-renders only the components that have actually changed. It does this efficiently and quickly, which is why React is known for its performance.

## 04. What is the difference between state and props?
**Ans:** State and props are both used to store data in a React component, but they serve different purposes and have different characteristics.

Props (short for "properties") are a way to pass data from a parent component to a child component. They are read-only and cannot be modified by the child component.

State, on the other hand, is an object that holds the data of a component that can change over time. It can be updated using the setState() method and is used to control the behavior and rendering of a component.

## 05. Can you explain the concept of Higher Order Components (HOC) in React?
**Ans:** A Higher Order Component (HOC) in React is a function that takes a component and returns a new component with additional props. HOCs are used to reuse logic across multiple components, such as adding a common behavior or styling.

HOCs are used by wrapping a component within the HOC, which returns a new component with the added props. The original component is passed as an argument to the HOC, and receives the additional props via destructuring. HOCs are pure functions, meaning they do not modify the original component, but return a new, enhanced component.

For example, an HOC could be used to add authentication behavior to a component, such as checking if a user is logged in before rendering the component. The HOC would handle the logic for checking if the user is logged in, and pass a prop indicating the login status to the wrapped component.

HOCs are a powerful pattern in React, allowing for code reuse and abstraction, while keeping the components modular and easy to maintain.

## 06. What is the difference between server-side rendering and client-side rendering in React?
**Ans:** Server-side rendering (SSR) and client-side rendering (CSR) are two different ways of rendering a React application.

In SSR, the initial HTML is generated on the server, and then sent to the client, where it is hydrated into a full React app. This results in a faster initial load time, as the HTML is already present on the page, and can be indexed by search engines.

In CSR, the initial HTML is a minimal, empty document, and the React app is built and rendered entirely on the client. The client makes API calls to fetch the data required to render the UI. This results in a slower initial load time, but a more responsive and dynamic experience, as all the rendering is done on the client.

## 07. How do work useEffect hook in React?
**Ans:** The useEffect hook in React allows developers to perform side effects such as data fetching, subscription, and setting up/cleaning up timers, in functional components. It runs after every render, including the first render, and after the render is committed to the screen. The useEffect hook takes two arguments - a function to run after every render and an array of dependencies that determines when the effect should be run. If the dependency array is empty or absent, the effect will run after every render.

## 08. How does React handle events and what are some common event handlers?
**Ans:** React handles events through its event handling system, where event handlers are passed as props to the components. Event handlers are functions that are executed when a specific event occurs, such as a user clicking a button. Common event handlers in React include onClick, onChange, onSubmit, etc. The event handler receives an event object, which contains information about the event, such as the target element, the type of event, and any data associated with the event. React event handlers should be passed 
as props to the components, and the event handlers should be defined within the component or in a separate helper function.

## 09. What are some best practices for performance optimization in React?
**Ans:** Best practices for performance optimization in React include using memoization, avoiding unnecessary re-renders, using lazy loading for components and images, and using the right data structures.

## 10. How does React handle testing and what are some popular testing frameworks for React?
**Ans:** React handles testing using testing frameworks such as Jest, Mocha, and Enzyme. Jest is a popular testing framework for React applications, while Mocha and Enzyme are also widely used.

In conclusion, understanding key concepts and principles of React is crucial for every React developer. This article provides answers to 10 important questions related to React including what is React, the virtual DOM, how React handles updates and rendering, the difference between state and props, Higher Order Components, server-side rendering and client-side rendering, and more. Understanding these topics will help developers to build efficient and effective web applications using React. 

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