Top ReactJS Interview Questions for Developers & Hiring Managers

Most Common ReactJS Interview Questions

ReactJS interview questions can take many shapes and sizes. Are you stressing about an upcoming interview? Do you need to interview a React Developer?

We all know that ReactJS interviews can be painful. An interviewer can ask questions from many areas of ReactJS. From old versions of ReactJS to the latest ones. And even though you have answers to many questions about React, it’s important to be prepared when you go to a ReactJS interview.

One challenge that everybody faces is preparing for the interview. Where do I get the right questions from? How do I answer them?

We’ve carefully compiled a list of commonly asked ReactJS Interview Questions and answers. No matter your role, these ReactJS Interview Questions will help you be more prepared.

Conceptual ReactJS Interview Questions

What is ReactJS?

React is a JavaScript library, it’s not a framework like Angular or Vue.js. You’re not going to get everything in React, like state management and routing.

In simple words, React is like a “V” of the “MVC” design pattern. It helps us design the user interface of our applications. 

Many people love that ReactJS is completely modular. It can be dropped into almost any web application.

What are React Components?

ReactJS introduces the concept of components. 

Components are basically a kind of code that we are going to reuse again and again. This enables you to design a modular application architecture.

Let’s suppose we are repeating some widget design throughout a web page. In this case, we should create a component to enhance the scalability of our design. In other words, any UI component that is used more than once should be represented as a component. The benefits of this include better development processes and faster updates.

What are the limitations of using ReactJS?

As you already know, ReactJS is a library, not a web development framework. That means it’s lacking certain features like routing and state management. 

In fact, sometimes these limitations can be very good for the React library itself.

In React, it’s just like you are writing logical HTML. You don’t have to write script.js for other things. You do have to manage your own components because react is built for Single Page Applications (SPA).

These are good for React because now you can build missing features according to your needs. For example, if I need to implement a router, I can build one on top of your existing ReactJS code. This also makes ReactJS compatible with other web development frameworks, like Angular and Vue.js.

Though, it’s these limitations that prevent your application from becoming bloated with packages for features that aren’t even used. For example, what if I only want to create a one-page application then I don’t need a router, so I don’t need to install that React router in my app.

What is the Virtual DOM?

The Virtual DOM is a JavaScript implementation of the real DOM. So basically, the React team implemented its own DOM model in terms of a whole JavaScript object.

React makes two copies of the real DOM. One maintains the exact copy of real DOM while the other represents the updated user interface. Both of them are known as virtual DOM.

What if you make some changes to the UI then React will put the changes on one of the copies and compare it with the other copy. If it finds a change, then it will automatically add those changes to the real DOM.

What is Reconciliation in ReactJS?

Reconciliation is an algorithm or a process through which React updates the DOM. Fiber is the most recent reconciliation engine and was released in ReactJS version 16.

What is the difference between State and Props?

React is completely built through components. In general, every component has its own state.

Suppose if I design a UI that has a header, a footer, and the main content area then I can divide that whole UI into three sections. Each section will now represent a separate component for the header, main content, and footer.

As I mentioned earlier that a component has a state, so now an important point to be noted is that a state is directly proportional to your view. For example, if the state changes then your view is also going to change.

The state is totally private to a component. But, props are not private to a component.

Let’s see what happens if we are going to break the main component into sub-components. In that case, we’ve to pass data to our lower-level/child components. This is when we have to use Props.

Both props and state objects. But, props are read-only. Whereas in the state, you can change data or even change the state according to your view. But in the case of props, you can only pass data.

ReactJS Interview Questions About Development

How is JSX utilized in ReactJS applications?

JSX is a combination of JavaScript programming language and XML. It allows us to integrate our HTML and JavaScript code in such a way that it is very easy to read and flexible enough to enhance its efficiency. JSX is used to create templates.

Example Code:

render()
{
	return(
		<p>
			Top <strong>ReactJS</strong> Interview Questions
		</p>
	);
}

What is the ReactJS Context API?

Context is a universal centralized data for your React application. It is designed to pass data inside child components. 

Many people use Redux instead of the context API because it has more features.

Event handling in HTML vs React?

There are a few key differences between how an event is handled in React.js than in HTML.

First of all, React uses a camelCase convention to write event names. On the other hand, HTML follows lowercase convention. This looks like onClick and onclick, respectively.

In React, you don’t use parentheses to invoke a function. Instead, the function name is wrapped in brackets.

To prevent default browser behavior in React, you must use preventDefault() function unlike in HTML, where a simple return false; can do the job.

What is React Router?

React Router is a flexible routing library that allows you to navigate the application and ensures that your current URL and the web page are both in sync with each other. 

React Router wraps the history library to manage browser interaction. 

The three main components of React Router are BrowserRouter, HashRouter, and MemoryRouter.

What is React Intl?

React Intl is a library that is used to integrate internationalization capabilities right inside our React application. It comes with an advanced API and some very useful components. 

React Intl is a part of FormatJS

There are two ways to format entities using React Intl.

Knowledge of ReactJS Interview Questions

Great, you made it! You should already feel more prepared for your interview. If you haven’t already, you should begin applying to ReactJS Jobs.

There are still a lot of ReactJS Interview Questions that are not covered. Our goal is to provide you with the most commonly asked ReactJS Interview Questions.

Did we miss something? Send an email to brian@angularjobs.com with the subject “ReactJS Interview Questions Feedback.” 

With your help, we can make this the best resource for the most commonly asked ReactJS Interview Questions!