Top 10 React js interview questions.

I have selected the top 10 major questions you should know as a React developer in 2023, whether you are interviewing for a hired position or not.

To get the best results out of this guide, make sure to try to answer each question yourself before looking at the answers.

Let's get started!

01. What is React and why is it useful?

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 is used by React to optimize the performance of updates to the UI by only updating the parts of the DOM that have changed.

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 a React component?

Ans: A React component is a JavaScript function or class that returns a React element, which describes the UI for a piece of the application. Components can accept inputs called "props", and manage their own state.

05. What is JSX and why is it used in React?

Ans: JSX is a syntax extension for JavaScript that allows you to write HTML-like elements in your JavaScript code. It is used in React to describe the structure of UI components.

06. 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.

07. What is the difference between controlled and uncontrolled components in React?

Ans: In React, controlled and uncontrolled components refer to the way that forms are handled. A controlled component is a component where the state of the form is controlled by React, and updates to the form's inputs are handled by event handlers. An uncontrolled component, on the other hand, relies on the default behavior of the browser to handle updates to the form's inputs.

A controlled component is a component where the value of input fields is set by state and changes are managed by React's event handlers, this allows for better control over the form's behavior and validation, and it makes it easy to handle form submission.

On the other hand, an uncontrolled component is a component where the value of the input fields is set by the default value attribute, and changes are managed by the browser's default behavior, this approach is less performant and it's harder to handle form submission and validation.

08. What is the role of Redux in React?

Ans: Redux is a JavaScript library that provides a centralized store for the state of an application and ensures that the state is updated in a predictable and consistent way, by using unidirectional data flow and actions/reducers paradigm. It allows developers to easily manage the state of complex applications and make the state management more predictable and easier to debug.

09. What is a Higher Order Component (HOC) in React?

Ans: A Higher Order Component (HOC) is a technique in React for reusing component logic. It is a function that takes a component as an argument and returns a new component that has additional functionality. HOCs are used for tasks such as authentication and authorization, providing props, or handling events. They allow for better code organization, separation of concerns, and overall code reuse.

10. What is the difference between server-side rendering and client-side rendering in React?

Ans: Server-side rendering (SSR) is when the initial render of a React application is done on the server, while client-side rendering (CSR) is when the initial render is done on the client. SSR allows for better SEO and faster initial load times, but requires a more complex server-side setup, while CSR allows for better user interactions and dynamic updates, but can have slower initial load times.