React.js is a versatile front-end library that’s widely used among JavaScript developers. It was developed in 2011 to create fast, responsive user interfaces. React.js developers work closely with back-end developers to build efficient, intuitive applications.
Using client-side routing, React.js speeds up responses to user requests by only fetching the sections of a web page that need to be changed instead of reloading the entire page for every request.
Skilled React.js developers should have a deep understanding of how the library works as well as extensive experience with other front-end web languages, including HTML, CSS, and JavaScript. Interviewing React.js developers can be challenging for hiring managers because it’s a relatively new technology that requires a narrow, specialized skill set.
This guide will help you conduct effective interviews by providing a broad range of questions you can ask React.js developers about their experience with the library, their approach to building UI components, and their ability to troubleshoot and debug during the development process.
React Interview Questions
If you're looking for an entry-level React developer, your main concern will be whether they have the basic skills to use React for front-end web applications under the supervision of more skilled mid- and senior-level developers who will mentor them as they continue to develop their expertise. These questions will cover the basic concepts even a beginning React developer should understand and be able to explain.
Can you explain the purpose of React and how it's different from other JavaScript libraries?
While this is an extremely basic question any applicant should be able to answer in their sleep, it's a good opener that should put your candidate at ease. It will also help you judge their communication skills in general. Developers sometimes prioritize hard skills and neglect soft skills. Look for a candidate who can communicate the following information in an easy-to-understand, helpful manner.
A good answer will include the following information: React is a JavaScript framework library for building user interfaces. Developers can use it to create reusable, easily shared, and managed components. Unlike some other JavaScript libraries, React focuses solely on rendering UI elements so it isn't a complete solution for all aspects of web development.
What are some of the major features of React, and what do they do?
React is unique in several ways. While this list isn't exhaustive, your candidate should hit on these highlights to demonstrate a solid grasp of the features and capabilities of React.
Components
Components are the building blocks of a React application. They are reusable pieces of code that represent a part of the UI. You can define components with JavaScript classes or functions.
JavaScript XML (JSX)
JSX is a syntax extension for JavaScript that allows you to write HTML-like code in your JavaScript files. JSX makes it easier to create and manipulate the DOM (Document Object Model) elements in a React application.
Virtual DOM
The Virtual DOM is a lightweight in-memory representation of the actual DOM. React uses the Virtual DOM to optimize the application's performance by minimizing how often you need to manipulate the DOM.
State
State is a JavaScript object that stores the internal data of a component. You modify the state with the command "setState()," and any changes to the state will re-render the component.
Props (short for properties)
Props are arguments passed to a component from its parent. You can use them to pass data from the parent component to the child component with the "this.props" object.
Lifecycle methods
Lifecycle methods are special methods called at different stages of a component's lifecycle, from the time it's created until it's destroyed. These methods are functions that you can define in a React component to run code at specific points in the lifecycle of a component.
How does the virtual DOM work in React, and why is it important?
The virtual DOM (VDOM) is one of the key advantages of React. This question will illustrate whether a candidate has a solid understanding of the benefits of the VDOM and why it's such an important part of React. Look for an answer that includes the following elements:
The (VDOM) is a lightweight in-memory representation of the actual DOM. For every object in the DOM, there is a corresponding object in the VDOM with the same properties. It serves as an intermediary between the React components and the actual DOM, allowing React to efficiently update the actual DOM while making minimal changes to the actual DOM.
The VDOM works by comparing the previous VDOM with the new VDOM and applying the minimal set of changes necessary to make the actual DOM match the new VDOM. This process is known as reconciliation.
The VDOM allows React to optimize the performance of updates to the actual DOM. It's much more resource-intensive to update the actual DOM than the VDOM. With a VDOM, React can avoid expensive DOM manipulation operations, such as traversing the DOM tree and updating individual DOM elements. Instead, React can simply update the virtual DOM and let the VDOM algorithm handle the actual DOM updates.
The VDOM also makes it easier to develop and maintain large, complex applications since it allows developers to write declarative code that describes how the UI should look rather than imperative code that specifies how to manipulate the DOM to achieve the desired UI. It's one of the reasons React is user-friendly — it creates a codebase that is easier to understand.
Can you explain the difference between a stateful and a stateless component in React?
Stateful and stateless components have different uses in React. A candidate should be able to tell you the differences and functions of each. In general, stateful components are used to manage a state specific to a single component, while stateless components present data and handle user input.
A stateful component is a component that maintains its internal state. This state can be modified over time, and the component will re-render to reflect these changes. A stateless component is a function-based component that does not maintain its state. It receives props from its parent component and renders itself based on those props, but it can't modify its state.
Stateless components are usually simpler and easier to test, and they can also be easier to understand because they don't have to manage the state. However, stateful components are necessary when managing a state that affects multiple components or persists across multiple interactions with the component.
Can you describe the lifecycle methods of a React component and give an example of when you might use each one?
As stated above, lifecycle methods are one of the major elements of React, giving it much of its functionality. Understanding the lifecycle methods — including what methods are used in each lifecycle phase — is an important skill for a React developer. A candidate's answer should include the following information regarding phases of a component's lifecycle and methods used during each one:
Mounting phase
This is the phase where a component is inserted into the DOM. The following methods are called during this phase:
constructor(): This method is called before the component is mounted and can be used to set the initial state of the component or bind event handlers to the component instance.
render(): This method is called to render the component to the DOM. It should return a description of what the component should look like.
componentDidMount(): This method is called after the component has been successfully rendered to the DOM. It can trigger an action after the component has been mounted.
Updating phase
This is the phase where a component's state or props change, and the component needs to be re-rendered to reflect those changes. The following methods are called during this phase:
shouldComponentUpdate(): This method is called before the component is re-rendered and can be used to optimize the component's performance by returning false if the component doesn't need to update.
render(): This method is called to re-render the component to the DOM with the updated state or props.
getSnapshotBeforeUpdate(): This method is called right before the updated component is committed to the DOM and can be used to capture the current state of the component.
componentDidUpdate(): This method is called after the component has been re-rendered to the DOM with the updated state or props. It can trigger an action after updating the component.
Unmounting phase
This is the phase where a component is removed from the DOM. The following method is called during this phase:
componentWillUnmount(): This method is called before the component is unmounted and can be used to clean up any resources or subscriptions the component has created.
React.js Advanced Interview Questions
When hiring an experienced React js developer, you want someone who has a deep understanding of advanced React concepts and can work as part of a team without direct supervision. Generally, advanced React developers will have at least five years of experience and have worked on many real-world projects they can explain in detail. Here are some questions and sample answers that can help you gauge an experienced developer's grasp of React:
What is strict mode in React?
Strict mode was added to React in version 16.3 to help identify potential problems inherent in the code. Since almost all code is built at least partially on third-party components, the danger of unexpected interactions is always present. Strict mode can help developers prevent problems before they become serious issues, so experienced developers should understand and use it.
You can enable strict mode by wrapping a block of code with the "React.StrictMode" component.
When strict mode is enabled, React will perform additional checks and warnings for:
- Deprecated methods or APIs
- Side effects in the component lifecycle methods
- Potential problems with the implementation of shouldComponentUpdate
- Re-rendering of components that have side effects
Strict mode does not affect the behavior of your code — it only adds warnings for potential problems. With more teams moving to a DevSecOps approach, Strict Mode is a valuable built-in tool in React that can help you catch potential issues early on — complying with the "security is everyone's responsibility" mantra.
What are some of the different ways you can style a React component?
There are many different ways to style React components, and an advanced developer should be able to use most of them.
Inline styling
Using the "style" attribute, you can apply inline styles directly to the JSX elements in your component. This can be a useful way to apply a small amount of styling, but it can become unwieldy as your styles become more complex.
CSS stylesheets
You can create a separate CSS stylesheet and link to it in your HTML file. This allows you to apply styles to your React components by assigning them class names. This method is good for separating your styles from your component logic and making your code easier to maintain.
CSS modules
CSS modules allow you to write CSS in your JavaScript files and apply the styles directly to your React components. This method can make it easier to manage your styles and avoid conflicts with global styles.
Explain the use of hooks in React
In React, a "hook" is a feature that allows you to use state and other React features in functional components instead of only in class-based components. React contains built-in hooks and allows you to create custom hooks. Advanced developers should be very comfortable using built-in hooks, including the following:
Basic hooks
useState: This hook allows you to add state to functional components. It returns an array with two elements: the current state value and a function that can update the state.
useEffect: This hook allows you to perform side effects in functional components. It is called after the component renders and can trigger an action, such as making an API call or subscribing to an event.
useContext: This hook allows you to access the React context in a functional component. The context is a global state that can be shared between components.
Additional hooks
useReducer: This hook is similar to useState, but it allows you to manage complex state logic using a reducer function. It returns the current state value and a dispatch function that can update the state.
useCallback: This hook returns a memoized version of a callback function. It's used to optimize the performance of components that rely on expensive functions.
useMemo: This hook returns a memoized value. It can optimize the performance of expensive calculations in a component.
useRef: This hook returns a mutable reference to a DOM element or value. It's used to access the DOM or store a value that persists between re-renders.
useImperativeHandle: This hook allows you to customize the instance value exposed to parent components when using the ref attribute.
useLayoutEffect: This hook is similar to useEffect, but it is called synchronously after all DOM mutations. It can perform DOM layout and paint operations.
useDebugValue: This hook allows you to display a label for a custom hook in the React DevTools. It's intended for debugging purposes only.
Custom hooks
A custom hook is a user-defined function that follows a specific convention for using and sharing stateful logic across multiple components. Custom hooks allow you to extract and reuse logic that would otherwise be written in multiple components. Custom hooks are an advanced feature in React, so you'd only expect experienced developers to understand and use them.
Soft Skills Interview Questions for React Developers
Although hard skills are certainly essential for React developers, don't overlook the importance of screening for soft skills. According to Deloitte, the line between soft skills and hard skills is blurring. As the rapid rate of technological advancement continues to increase, possessing hard skills will continue to become less important than the ability to learn new skills as old ones become quickly outdated.
Regardless of how skilled a React developer is, they won't bring much to your organization without effective communication and collaboration skills. Given how quickly skills need to be updated, it's a good idea to focus on finding a candidate who is dedicated to continuous learning, works hard, is a creative problem-solver, and gets along well with others. This candidate might be a better bet than the one who knows the most about React. Of course, if you can find all of those qualities in one candidate, you should hire them on the spot.
Here are some questions you can ask to learn more about a candidate's soft skills. While you'll want to pay attention to what they say, you should also hone in on how they communicate. Remember that they're bound to be nervous, so look beyond what may simply be a case of the jitters from being put on the spot.
How do you stay current with new technologies and programming languages?
There are numerous ways candidates can stay up to date on technology trends, including forums, magazines, blogs, conferences, and professional organizations. Look for candidates who are clearly on top of new and emerging technologies and enjoy learning new skills. They'll be valuable to your organization long after current React skills are obsolete.
How do you handle challenges or roadblocks in a project?
Obstacles and frustrations are everyday events for programmers. If they don't have an unflappable attitude toward the challenges that pop up and a clear method for dealing with them, they'll spend a lot of time miserable and vexed — not a good state of mind for creative problem-solving. Ask them to tell you about their most difficult challenge and how they overcame it.
How do you handle conflicts or disagreements with team members or stakeholders?
Like obstacles, conflict is an inevitable part of life for developers. Many roads lead to Rome, and there are bound to be disagreements about which is best. Being able to peacefully work out conflicts while still getting your ideas across is essential for developers. Effective communication skills can help tremendously, particularly with stakeholders who may not understand the technical details. Developers need to be able to explain the processes and outcomes without relying on overly technical jargon that will only confuse lay people.
How do you handle tight deadlines or heavy workloads?
Project management is another crucial aspect of deploying software. You want a developer who is comfortable using project management tools and can break large tasks into smaller, more manageable steps to help them adhere to a timeline. They should also be able to communicate effectively with other team members regarding changes, updates, and work performed. With remote teams now the norm, good asynchronous work habits are vital to a project's success.
Even if your teams are working together, everyone needs to contribute to status updates so they're all on the same page regarding what's been done and the next priority. Although the team leader will ultimately be managing the product timeline, effective project management relies on everyone's participation.
Hire React js Developers
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