How to Interview a React.JS Developer and What to Look For?

React continues to be one of the most widely used and beloved app development libraries, offering both significant benefits and challenges. Its popularity, fueled by React's simple declarative API and flexible application capabilities, has made the developer market highly competitive, especially when it comes to attracting top React.JS talent.

If you want to build reliable, robust, and visually appealing web apps using proven technologies, React is an excellent choice. However, finding skilled React.JS developers who can help your app stand out in a crowded marketplace is crucial. Identifying top-tier talent requires more than just sifting through resumes; it begins with a strong recruitment process.

In our article "A Comprehensive Guide to Hiring React.JS Developer Teams," we break down the steps to finding the right developers and provide insights into the qualities and skills to look for during interviews. By asking the right questions and analyzing the answers, you'll be able to uncover the knowledge and expertise that set great developers apart from the rest.

In this guide, we outline key interview questions, example responses, and qualities to help you identify the perfect candidate for your development needs. If you're looking for further assistance, we can connect you with the right company. Just tell us what you need, and within 72 hours, we'll match you with up to 5 companies—all for free.

Responsibilities of a React.JS Developer

Before you dive into asking who you want to hire, it’s worth looking carefully at what qualities your ideal developer will have. Understanding the qualities you need will help guide your decision making to land the precise candidate you’re looking for.

Write Clean and Maintainable Code

The first and foremost skill of any developer. Clean, clear, and easily understood code should be number one on the list of priorities.

This means code that is easy to read, well understood, and able to be modified by someone other than the developer who first wrote it. A developer who writes clean code saves time on scaling, maintenance, and debugging for weeks, months, and years to come.

Participate in Every Stage of the Development Lifecycle

While not necessarily an expert in every area, an experienced React developer should have some input at every stage of application development. Expressing an opinion in all phases, from initial planning to deployment and support, can prevent issues from arising later. Developers with a great deal of experience in personal projects may even have practical knowledge and experience in every phase.

Asking about side projects during the interview can be a great way to find out what experience a developer has alongside their professional credentials.

Strong Problem Solving and Critical Thinking Skills

A major part of application development is practical problem-solving. This should extend outside of just working code into the code of underlying libraries and systems that support it. A developer with strong problem-solving skills can save hours and days from a single project without even realising it.

Awareness of Current Trends and Technologies

The software industry moves fast; it’s a practical requirement for developers to keep on top of what’s changing and what updates are happening within it.

This know-how shouldn’t necessarily be complete, by any means, but a general awareness of available tools and technologies is a valuable asset to have. An opinion on the advertised features and potential for such tools is even better still.

If you are preparing for an interview and need to brush up on recent trends in React.JS, we have you covered with our article on the future of React.JS development.

Generalised Interview Questions and Techniques for React.JS Developers

Do you Have a Passion Project?

Starting off the interview with an easy question can be an ideal icebreaker. It helps calm inevitable pre-interview nerves and serves as a jumping-off point to frame questions about software development principles and practices.

The answer a candidate gives to this question can help shape the rest of the interview, providing a reference point against which to evaluate technologies.

Code Analysis

Before a candidate has come in to interview, you should have already looked at code available either as part of the application or available publicly in a Git repository or similar.

Existing code already in use is the best possible view you’ll get of how well a candidate will write code in the future. You can get into even greater detail by asking candidates to talk through features they have implemented or choices they’ve made when building web apps.

Describe how you would tackle the development of a React web application from requirements through to delivery

With this generalised starter question, you should begin to get a good feel for an applicant’s experience and expertise. It’s a valuable window into how they view the process of building a React.JS app and the areas they pay more attention to than others.

A good answer should include a reasonably detailed explanation of requirement analysis, specification writing, planning, development, test, and deployment. If you want a more detailed answer, it would be reasonable to dive further into this question, perhaps asking for a breakdown of how much time to allocate to each. You might also ask about common problems you have faced during development, such as changing requirements or a delivery date brought forward, and ask how this would change things.

What to Look for in a React.JS Developer

React.JS is a highly popular technology amongst both professionals and enthusiasts. Like other highly popular technologies out there, that means it’s relatively easy to pick up and try out. Professional use, in skilled hands, takes much more time and effort.

The skills and experience of various React.JS developers may be somewhat varied as a result. Even two highly qualified React developers are likely to have taken very different paths to get there.

There are a core set of skills expected of any React developer at a professional level.

Hard skills of a React.JS developer should include:

• Expert knowledge of JavaScript in a production environment. Writing good JavaScript code is essential to understanding the underlying architecture
• Expert knowledge of related components, tools, and frameworks. Developers with more than a passing experience with React will be familiar with tools such as React Router, Redux, or Material Kit React
• Detailed understanding of React’s concepts and structure. Understanding the Virtual DOM, JSX, and component lifecycle are all essential to get up and running as a React.JS developer
• Strong background in ES6, HTML/CSS. These are essential components in front end development
• Experience in unit testing, Git, and Continuous Integration(CI)/Continuous Delivery(CD) tools. Each has an essential role to play in developing modern apps
• Experience with REST API/GraphQL. Something you’d expect of React.JS developers for integrating with third-party platforms

Soft skills of a React.JS developer should include:

• Strong communication skills and an open-minded approach to working with others. One of the most overlooked abilities of skilled software developers is the ability to communicate ideas well and accept the opinion of others
• Desire to learn. The software industry moves fast, and its tools are consistently improving. Good developers should always be looking to improve their skill-sets and increase their value and abilities
• Transforming requirements and goals into technical specs and code. An ability that pays dividends in being able to work on applications from the initial stages through to completion

Skills Questions for React Developers

Implement linear search in JavaScript

This question tests both knowledge of data structures and algorithms and demonstrates some basic knowledge of JavaScript too.

function linearSearch(array, key){

let results = []

for(let i = 0; i < array.length; i++){

if(array[i] === key){

results.push(i)

}

}

if(!results){

return -1//empty array

}

return results

}

}

Describe how you’d build a single page application with multiple sections using React.JS

Another short and simple test that demonstrates an understanding of the technologies you’re interested in. If the candidate has access to a computer, you may ask them to implement the page instead. Doing this as a pair programming exercise can be a great, if stressful, way to see how candidates work under pressure.

Describe some steps you might take to improve the performance of a web page

Most candidates should have some ideas to bring to the table to improve loading times. Some of these might include:

• Using adaptive images
• Compressing content
• Cache improvements
• Combining images into CSS sprites
• Enabling HTTP Keep-alive response headers
• Minify Javascript and CSS

Candidate Technical Interviews

Technical interviews, in software development, are notorious both inside and outside the industry for including gotcha questions and irrelevant exercises. Conducting a good interview should mean avoiding both of these.

Employing the tactics used in the early 2000s does little more than give a bad impression of your business, makes your environment look like one where candidates won’t want to work even if they do get an offer, and teaches you very little, if anything, about strong candidates.

You can find out a great deal more about the right candidate with nothing more than just discussing the concepts and ideas that a react developer should know.

The best candidates, the ones that will fit in well and create the most productive employees, can articulate their opinions on technical topics and draw from experience to inform their answers. The perfect candidate will be able to take on board the opinions of others and move the discussion forward accordingly.

Alongside interview questions, in-depth programming exercises are another excellent way to allow a candidate to demonstrate their own skills and experience.

When it comes to the interview, honest discussion with candidates is worth ten times that of a formalised quiz or interrogation. The questions below should simply help to inform that discussion, acting as a jumping-off point to go into each applicant’s personal experiences of the tools and technologies.

Doing things this way, you’ll find out far more about both soft and hard skills—figure out where they might fit in well and maybe even learn a thing or two about React.JS development in the process.

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React.JS Interview Questions to ask Candidates

What is React.JS?

React is an open-source JavaScript library for building complex, responsive user interfaces.

How is React different from other JavaScript Frameworks?

A candidate’s own experience with other JavaScript frameworks will inform their answer to this question. The core of an answer should include React’s focus on the creation of components and the agnostic relationship it has to an app’s underlying architecture. The listed advantages might talk about the additional flexibility React provides developers.

A good follow-up question would be to ask when they may choose React over one of the other technologies they’ve used in the past and why.

Describe the lifecycle of a React Component

The component lifecycle is the key part of React. Broadly, components have lifecycle events in three main stages:

• Initialisation
• State updates
• Destruction

What is JSX?

When React was first launched, it simultaneously launched with JSX—a syntax extension to JavaScript created by Facebook.

Developers do not have to use JSX to write React applications, but it has become a de facto standard within React.JS. A candidate with sufficient experience should be able to talk about the pros and cons well.

What are Stateless Components?

State is a critical component of React so stateless components can seem somewhat strange.

Stateless components are simply pure functions that render DOM based on only the properties passed to them.

What is the StrictMode Component, and why would you use it?

StrictMode is included with React to highlight any potential issues within components. Potential issues are logged to the console in development mode to alert the developer of any issues. In production mode, these warnings are not shown at all.

Developers use StrictMode to stick closely to best practices and maintain high-quality code through the development phase.

What do you know about the Flux pattern?

Using Flux, the data store is a central authority for all data. Data changes must occur within the store from which they are broadcast to view via events.

Actions may be fired, controlled by a central dispatcher, to request changes in-store data. The advantage that this model has is that data stability is ensured and runtime errors are minimised.

List some common approaches to Styling a React.JS application

• CSS Classes
• Inline CSS
• Pre-processors (Sass, Stylus, less)
• Css-in-JS modules such as Emotion, Styled-jsx, and styled components

Explain the standard JavaScript toolchain

To unfamiliar developers, the JavaScript toolchain can seem overly complex. It’s important to have a good mental picture of how these pieces fit together.

A good answer should include:

• Dependency Management
• Linting
• Style-checking
• Transpilation
• Compilation
• Minification
• Source-Mapping.

What Tools might you use to improve the accessibility of a React application?

Static analysis tools:

Linting tools can be used to analyse React projects at the component level. These are low-cost, easy to use, and run very quickly early in the development phase. These should typically be the first line of defence against accessibility issues appearing.

Browser tools:

At the application level, browser accessibility tools such as Google Lighthouse can analyse accessibility to discover real-world issues. Ideally, these tools would be run in a CI/CD environment to pick up issues after the app has been deployed to test.