32 ReactJS Interview Questions and Answers

32 ReactJS Interview Questions and Answers for 2019

Whether you’re an entrepreneur, a project manager in a large enterprise, or a CTO, you are well aware that the success of your project rests on your ability to find top developers.

In this guide, you‘ll find example interview questions and answers you can refer to when seeking a new ReactJS developer to make your dynamic user interfaces come to life. You‘ll also get some practical advice on how to use these questions to reliably identify the best developers for the job.

First Things First: Select Your Job Requirements

ReactJS is a flexible JavaScript library that can be used in all different kinds of applications and alongside other web technologies. To find a ReactJS that‘s a great fit for your project, you need to get clear on what your job requirements are.

Think about the type of application you are developing, and what technical skills are critical to its success. You also need to think of the softer skills that a candidate needs to work well with the rest of your team.

Some example ReactJS job requirements include:

  • Essential web development skills E.g. HTML, Javascript
  • ReactJS-specific skills E.g. JSX
  • Library/toolkit experienceE.g. Redux/MobX
  • Design skillsBuilding scalable applications, using the Flux architecture
  • Communication skillsDiscussing problems and constraints with your clients
  • Being a self-starterIf you don‘t want to do any hand-holding

Avoid making a laundry list of non-essential skills for your perfect ReactJS developer. Instead, focus on what your candidate will actually be doing day-to-day. Keep your requirements list as short as possible. Cut anything they can do without or learn on the job.

With clearly stated requirements, you‘ll be able to choose the right ReactJS coding interview questions and have a much better idea of what kinds of responses you are looking for.

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In this guide, we categorize the questions based on the level of expertise you‘re looking for. Junior, Mid-level, and Senior. If you‘re unsure exactly which one you need, take a look at the different sections for a summary of what you can expect from each.

ReactJS Junior Interview Questions

Junior developers have the least experience and demand the lowest salary. It‘s important to note before hiring a junior that they need to work under the guidance of more experienced developers.

Skill Requirements for Junior ReactJS Developers

  • Basic HTML, CSS and Javascript skills
  • Foundational ReactJS knowledge
  • Learning on the job
  • Following instructions and receiving feedback
  • Thinking like a programmer

Example ReactJS Basic Interview Questions and Answers

Note: Important keywords are underlined in the answers. Bonus points if the candidate mentions them!

Or save yourself time and request a team to match your needs right away.

Question 1: Write an example of a simple HTML document with some header information and some page content.
Requirement: Basic HTML skills

Answer:  HTML documents are different, but they follow a basic structure of head and body. The different sections are marked with tags such as: DOCTYPE, html, head, title, meta, body, h1, p.

For example:

     <!DOCTYPE html>
     <html>
       <head>
         <title>Page Title</title>
         <meta charset="UTF-8">
         <meta name="description" content="Page description">
       </head>
       <body>
         <h1>Interview Example Web Page</h1>
         <p>Some content goes here</p>
       </body>
     </html>

Question 2: What is the purpose of the following code?
Requirement: Using CSS with React

    render() {
      let className = 'menu';
      if (this.props.isActive) {
        className += ' menu-active';
      }
      return <span className={className}>Menu</span>
    }

Answer:  This code adds a CSS class to a component by passing a string as the className prop. Here the class depends on the component props or state which is common with ReactJS.

Question 3: Explain what a Javascript callback function is and provide a simple example.
Requirement: Basic Javascript skills

Answer: A callback function is a function that is called after another function has finished executing. A callback function is passed to another function as an argument and is invoked after some operation has been completed. For example:

     function modifyArray(arr, callback) {
       arr.push(100);
       callback();
     }

     var arr = [1, 2, 3, 4, 5];

     modifyArray(arr, function() {
       console.log("array has been modified", arr);
     });

Question 4: Briefly describe ReactJS in one or two sentences.
Requirement: Foundational ReactJS knowledge

Answer: ReactJS is an open-source Javascript library. It is designed for building dynamic single page applications that require less coding than doing everything yourself in Javascript. Sometimes it is called a framework, but that‘s a question of terminology.

Question 5: What is ReactJS? How does it compare to other JavaScript frameworks?
Requirement: Foundational ReactJS knowledge

Answer: Here you are getting a feel for the candidate‘s knowledge, ideas and opinions of what React is, and how it fits into the JavaScript ecosystem. Some points to look out for:

  • React is an open-source JavaScript library maintained by Facebook
  • It‘s designed for building dynamic and interactive UIs (user interfaces) for web and mobile applications
  • It is used to help you build UI components
  • Sometimes seen as the ’View‘ in the ’Model-View-Controller‘ (MVC) architecture.
  • This means it is a relatively small framework (compared to, say, Angular, which takes care of the whole front-end) and can be used easily with other libraries and integrated into many applications
  • React isn‘t as opinionated about an application‘s architecture as other frameworks like Angular and Vue.js
  • Very strong community with many supporting tools and packages

Question 6: How would you learn about a new Javascript library?
Requirement: Learning on the job

Answer: Web development is changing all the time, and developers need to be able to learn constantly. Here you are finding out how the candidate approaches the learning process. You want to see that they have an idea of what information they will need and where to find it. For Javascript libraries, that means looking up online tutorials and digging into the documentation.

Question 7: Describe a time you received feedback on a programming task. How did it help you become a better programmer?
Requirement: Following instructions and receiving feedback

Answer:  This is a typical open-ended question. The candidate should demonstrate they can accept, understand and act on feedback.

Question 8: A common type of problem we have to solve at this company is _____. How might you think about finding a solution?
Requirement: Thinking like a programmer

Answer: In this question, the problem should be directly related to the work the candidate will actually be doing day-to-day working for you. You aren‘t looking for a perfect answer or even necessarily a correct answer. Instead, listen to how they approach solving a problem, their ability to break a problem down into parts, and if they can anticipate problems.

ReactJS Mid-Level Interview Questions

Mid-level developers are the workhorses of the software development world. They are fairly experienced and you can rely on them to execute routine tasks with skill and efficiency. They can also mentor junior developers.

However, if you need a developer with high-level design skills to lead your project, you might want to skip down to the expert-level ReactJS programming interview questions.

Note: Important keywords are underlined in the answers. Look out for them in interviews!

Question 9: List some advantages and limitations of ReactJS?
Requirement: Expert ReactJS knowledge

Answer:

Advantages:

  • Free and Open Source
  • Highly adaptable
  • Virtual DOM makes it extremely efficient
  • JSX increases code reliability and makes mixing HTML and JavaScript much easier
  • Makes UI testing easier
  • Developer tools such as the React.js chrome extension for debugging
  • More SEO friendly that some other JavaScript frameworks
  • Large supporting community

Limitations:

  • Library, not a framework. It handles the UI only. You need to use other libraries for other parts of your application
  • No set way to structure applications, meaning you have to figure it out for yourself
  • Its flexibility can easily let developers make poor choices
  • JSX and inline templating can make the code quite complex, especially for novice developers
  • A different way of thinking and learning curve compared to other frameworks like Angular

Question 10: Explain what the Virtual DOM is in ReactJS and why it is necessary.
Requirement: Expert ReactJS knowledge

Answer: The Virtual DOM is one of the key concepts in React. A good candidate should be able to explain the problems with DOM manipulation and why the virtual DOM helps.

DOM (Document Object Model) manipulation is how web applications update the HTML on a webpage to make it dynamic and interactive. However, updating the DOM is slow and most JavaScript frameworks update it more than they need to. Usually, by updating the entire DOM every time a small change is made on a page.

React tries to reduce this waste by only updating the parts of the DOM that are actually updated. To do this, React keeps track of a corresponding lightweight ’Virtual DOM Object‘ that can be updated and checked for changes much faster than the real DOM.

React uses Virtual DOM snapshots to work out exactly which parts of a page need updating, and updates only those in the real DOM rather than creating a new DOM. By using a virtual DOM, web applications can be made much faster and more efficient

Question 11: What is JSX? How does it relate to ReactJS? Give a quick example.
Requirement: Expert ReactJS knowledge

Answer: JSX (JavaScript XML) is a preprocessor that allows you to include XML syntax in your JavaScript. Its basic function is to make code more intuitive and easier to read. The React library realizes that often the JavaScript and HTML are strongly related, and having them in separate HTML and JavaScript files makes things confusing. Here are some examples of some JSX code:

     const element = <h1>Hello, world!</h1>;

     render() {
       return (
         <div>
           <h1>”Hello, World!”</h1>
         </div>
       );
     }

It isn‘t mandatory to use JSX with React, but it makes a lot of sense to do so.

Question 12: What are components in React?
Requirement: Expert ReactJS knowledge

Answer: ReactJS is component-based. That means that components make up the building blocks of a ReactJS application by splitting the UI up into many separate, reusable pieces. Components can be thought of like JavaScript functions. They accept inputs called props and return React elements that describe what should be presented to the user.

Question 13: What do the following two ReactJS code segments do?
Requirement: Expert ReactJS knowledge

1:

     function Welcome(props) {
       return <h1>Hello, {props.name}</h1>;
     }

2:

     class Welcome extends React.Component {
       render() {
         return <h1>Hello, {this.props.name}</h1>;
       }
     }

Answer: Both of the above define components in React. The first is a functional component and the second uses an ES6 class to define a component. However, from the perspective of ReactJS, the two above components are exactly equivalent.

Question 14: What does render() do?
Requirement: Expert ReactJS knowledge

Answer: Every React component has a render() function. Render() returns exactly one React element that represents a native DOM component. Multiple HTML elements must be grouped inside one tag, for example, <group>.

Question 15: What‘s your experience with the Flux architecture pattern?
Requirement: Experience using Flux architecture

Answer: Many ReactJS applications use Flux architecture rather than MVC. Its key feature is that it enforces unidirectional data flow. The three major parts of a flux application are the:

  1. Dispatcher
  2. Stores
  3. View (React components)

Question 16: What‘s your experience with the Redux library?
Requirement: Experience using libraries you use

Answer: Redux is a state management library commonly used with React. You‘re checking if your candidate can talk about using Redux (or another library you use) to build testable applications that can run across different development environments with predictable behavior.

  1. Single source of truth
  2. State is read-only
  3. Changes are made with Pure functions

Question 17: What are some testing tools you would for unit testing in a ReactJS application?
Requirement: Unit testing UI components

Answer: Unit testing is a technique to test that isolated segments of code are functioning properly. Some tools for testing ReactJS applications include Enzyme, Jest, react-testing-library, React, unit, Skin-deep, Unexpected-react.

Question 18: What is the children prop?
Requirement: Understanding JSX and composition.

Answer: A special prop that takes data between tags. For example:

     <Header>Hello world</Header>

props.children in component Header will be equal to “Hello world”.

Question 19: Can you describe the React Component lifecycle? When you should make an asynchronous call?
Requirement: React component lifecycle. React 16/17 breaking changes.

Answer:

The usual component lifecycle looks like this:

  1. Mounting
    1. constructor
    2. componentWillMount (should be mentioned, that in React 16/17 this hook is deprecated and replaced by getDerivedStateFromProps)
    3. render
    4. componentDidMount
  2. Updating
    1. componentWillUpdate (in React 16/17 it was replaced by getDerivedStateFromProps)
    2. shouldComponentUpdate
    3. render
    4. getSnapshotBeforeUpdate
    5. componentDidUpdate
  3. Unmounting
    1. componentWillUnmount

Before React 16 it was ok to make a call in componentWillMount. But now that componentWillMount has become deprecated the earliest hook for asynchronous call is componentDidMount. It‘s also ok to make calls from ’componentDidUpdate‘.

Example ReactJS Expert Interview Questions

Now for the most difficult interview questions and answers for experienced ReactJS developers. These are for finding the developers that can design and build you a world-class React application.

Skill Requirements for Senior ReactJS Developers

  • Expert ReactJS knowledge
  • Flux architecture
  • Designing for specific requirements (e.g. security, scalability)
  • Asynchronous programming
  • Maintaining and upgrading applications
  • Experience in other JavaScript frameworks and libraries (e.g. Node JS)
  • Efficient programming and clean code
  • Debugging
  • End-to-end and unit testing
  • Leadership skills
  • Clear communication skills
  • Mentoring less experienced developers

Question 20: What React patterns do you know? Can you describe what is HOC or Render prop?
Requirements: React patterns and app architecture

Answer: There are a lot of patterns for React, such as:

  • High Order Component (HOC)
  • Render Prop
  • Function as children
  • Container and Presentational components

HOC is the acronym of High Order Component, which is very similar to High Order Function. It‘s a function takes a component and returns new component with some changes. For example, Redux connect functions is the HOC, which takes a component and returns a component with Redux State and Actions.

Render Prop is the pattern for creating components with “render callback”. Usually, it is used for the same purpose as HOC. For example, it is used by React-Router 4 and Context API.

Question 21: Do you have any experience with React 16 Context API? What problems does it solve?
Requirements: React Context API

Answer: Context API provides a way to pass data through the component tree without having to pass props down manually at every level. It can be used to share global data (such as user, theme or any other information) between child components.

Question 22: What is React Portal and React Fragment? What problems do they solve?
Requirements: React Portal and React Fragment.

Answer: Portals provide a first-class way to render children into a DOM node that exists outside the DOM hierarchy of the parent component. For example, it can simplify Modal window rendering or changing information inside <head></head> tags.

React Fragments are for grouping a list of children without adding extra nodes to the DOM. Because React forces us to return only one object from render, we should wrap children into div or a similar container tag. Sometimes it‘s simpler to return several tags without a container to implement some features.

Question 23: What is React Reconciliation? How do you avoid performance issues?
Requirements: React rendering algorithm

Answer: Reconciliation is the process that React uses to efficiently update the DOM. It does this by only updating the parts that need it. At a single point in time, the render() function will create a tree of React elements. On the next state or props update, that render() function will return a different tree of React elements. React then needs to figure out how to efficiently update the UI to match the most recent tree. The process of figuring this out is reconciliation.

To avoid performance issues we should:

  • Add ’key‘ prop
  • Use PureComponent with React.memo()
  • Memoize functions (reselect, for example).

Question 24: What is the ’PropTypes‘ library?
Requirements: PropType library, data validation

Answer: PropTypes is the type-checking addition to React Library, which exports a range of validators to make sure the data component receive is valid. Using it is a good idea, because it reduces the number of bugs and makes component self-documented.

Question 25: Do you have any experience in code splitting with React? Can you describe how a bundle can be split into smaller chunks?
Requirements: Webpack bundling, React.lazy or React Loadable

Answer: React has tools to avoid large bundle sizes. You can use React Loadable, because of its simplicity, but React 16.6 also has solutions such as React Lazy.

As for React Loadable is a small library for wrapping components into bundles. It provides nice features such as placeholders for “Loading” and “Error” components and flash delay. When a component is wrapped, Webpack will move it into a new bundle.

Open-Ended Questions

Once you‘ve established that your developer is an expert in ReactJS, you should continue the interview by asking some messy and open-ended questions to spark a discussion. Tailor these questions to fit your own job requirements and don‘t be afraid to ask follow up questions.

Question 26: If you could use whatever tools you like to build our ____ application, what would you use?
Requirement: Design skills, understanding requirements

Answer: In this question, you are getting a feel for the type of developer you are talking to and how they like to code. You are especially looking for developers that try to understand the requirements first. It‘s a big red flag if a developer gives a list of libraries and tools without understanding the task.

Question 27: How are you involved in the ReactJS community?
Requirement: Passion for web development

Answer: This is a popular question for coding interviews because community and open source involvements are clear indicators of a passionate developer.

Question 28: Describe a time you fixed an error in a web application. How did you approach the problem? What debugging tools did you use? What did you learn from this experience?
Requirement: Debugging, Breaking down a problem into parts

Debugging is one of the key skills for any web developer. However, the real skill is in breaking the problem down in a practical way rather than finding small errors in code snippets. Debugging often takes hours or even days, so you don‘t have time in an interview setting. Asking these questions will give you an idea of how your candidate approaches errors and bugs.

Answer: In the candidate‘s response you should look out for things like:

  • A measured, scientific approach
  • Breaking down the problem into parts
  • Finding out how to reproduce the error
  • Expressing and then testing assumptions
  • Looking at stack traces
  • Getting someone else to help/take a look
  • Searching the internet for others that have had the same problem
  • Writing tests to check if the bug returns
  • Checking the rest of the code for similar errors
  • Turn problems into learning experiences

Question 29: What’s the most important thing to look for or check when reviewing another team member’s code?
Requirement: Mentoring less experienced developers, Leadership skills

Answer: Here you‘re checking for analysis skills, knowledge of mistakes that less experienced developers make, keeping in mind the larger project and attention to detail.

A good answer might mention code functionality, readability and style conventions, security flaws that could lead to system vulnerabilities, simplicity, regulatory requirements, or resource optimization.

Question 30: What tools & practices do you consider necessary for Continuous Integration and Delivery for a web application using ReactJS?
Requirement: DevOps systems design, Maintaining and upgrading applications

Example Answer: One example for a React applications is create-react-app. It provides a lot of features for building. It also good to separate config files for development and production.

As for deployment, it depends what hosting we are using. For example, for Firebase there is nothing complicated: usually we run build script and run firebase deployment script. Same for github pages, heroku, netlify.

For deployment to basic hosting with client-side rendering they might mention nginx and node server.

Question 31: What is your favorite method to fill a React Component with CSS? Do you have an experience with CSS-In-JS libraries?
Requirement: React component styling

Answer: There are several options to style components:

  • by className
  • by inline styles
  • CSS-in-JS
  • 3rd party libraries for styling components such as StyledComponents, Radium, CssModules, etc.

Question 32: What is the purpose of React Refs? Can you describe a problem you can solve by using it?
Requirement: React DOM interactions.

Answer: Refs is the method to access a DOM node or React component inside a parent component. A common problem it solves is to focus input or get child component‘s fields. It‘s better to use refs as little as possible.

Summary

Hiring the right people for your development team is critical to the success of your project. Remember that you should not be aiming to hire the perfect Java developer, but rather the right person for the job at hand.

With the help of this information and sample interview questions on Java, you can ensure that the hiring process goes smoothly and successfully – allowing you to design hire a great programming team to get your project completed on time and on budget.

Finally, here is a BONUS for you:
Click here to download a ready-to-use cheat sheet of questions and answers that you can simply print out and bring with you to the interview your candidates.