Top React.Js Interview Questions and Answer

React JS is a free and open-source front-end JavaScript library for building user interfaces based on components in web applications. React JS is developed and maintained by Facebook (Meta) and a community of individual developers and companies. React JS enables developers to create reusable UI components. React JS makes it simpler to manage the state and data flow in large applications. React is used to develop single-page, mobile, or server-rendered applications with frameworks like Next JS. React is only concerned with the user interface and rendering components to the DOM, React applications rely on libraries for routing and other client-side functionality. Understanding its core concepts such as components, JSX, and the virtual DOM is crucial if you're diving into React JS technical interviews.

One of the main benefits of using React JS is its potential to reuse components. It saves time for developers as they don't have to write various codes for the same features. Furthermore, if any changes are made in any particular part, it will not affect other parts of the application.

 

React provides users with an ample number of advantages when building an application. Some of them are as follows:

• With React, UI testing becomes very easy.
• React can integrate with Angular and other frameworks easily.
• The high readability index ensures easy understanding.
• React can be used for both client-side and server-side requirements.
• It boosts application performance and overall efficiency.

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.

 

The Virtual DOM is a lightweight in-memory representation of the actual DOM elements. Virtual DOM’s primary purpose is to boost the efficiency and performance of updating the UI in web applications. React creates a Virtual DOM to track changes instead of making direct changes to the real DOM. React then compares the current Virtual DOM with the previous one and calculates the best way to make these changes. This process is known as "reconciliation". Updates to the real DOM are batched and executed in the most efficient manner, leading to optimized performance and a smoother user experience.

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.

The five main building blocks of React are:

• Components: These are reusable blocks of code that return HTML.
• JSX: It stands for JavaScript and XML and allows you to write HTML in React.
• Props and State: props are like function parameters and State is similar to variables.
• Context: This allows data to be passed through components as props in a hierarchy.
• Virtual DOM: It is a lightweight copy of the actual DOM which makes DOM manipulation easier.

JSX is a syntax extension of JavaScript. It is used with React to describe what the user interface should look like. By using JSX, we can write HTML structures in the same file that contains JavaScript code.

In general, browsers are not capable of reading JSX and only can read pure JavaScript. The web browsers read JSX with the help of a transpiler. Transpilers are used to convert JSX into JavaScript. The transpiler used is called Babel