Advanced React Interview Questions (with answers) That Matter

React is one of the most popular front-end development frameworks at the moment. This means that the demand for React developers is high. In turn, this has led to thousands, if not hundreds of thousands, of developers learning and mastering React.

If you’ve not searched for React jobs recently, here’s something for you to consider — the competition for React jobs is ridiculously high. All this leads us to the interview process. Thanks to the sheer number of applicants for React jobs, the interview process is now pretty tough. No one’s going to ask you basic questions anymore.

This blog will not list 100 questions that no one’s going to ask. Instead, we’re going to look at 10 advanced React interview questions that you can expect an interviewer to ask.

Because we’re a job board, we get to see the recruitment process quite closely. It’s from these interviews that this blog has been compiled. To be clear, we expect the concepts covered in this blog are going to form a major chunk of your technical interview.

For each question, you will see three sections.

1. Why is this question asked?

2. An example answer

3. Why is this a good answer?

If your experience does not exactly match our sample answer, it’s absolutely fine. Just take a look at the third section and tailor your answer to match your experience.

Let’s get started.

10 Advanced React Interview Questions

Explain the diffing algorithm in React.

Why is this question asked?

The diffing algorithm in React is a fundamental aspect of the library's efficiency and performance.

This algorithm is the crux of how React decides to make changes to the DOM, which can have a huge impact on the user experience of a web application.

By understanding this algorithm, developers can better predict the behavior of their applications, optimize rendering, and improve performance.

A good understanding of the diffing algorithm can help in troubleshooting performance issues and designing efficient components. It also reflects a developer's competence beyond the basics of using React.

Example answer:

React uses this algorithm to efficiently reconcile the Virtual DOM with the Real DOM. This algorithm is part of React's re-rendering process, which happens whenever there's a change in a component's state or props.

The diffing algorithm works by comparing the new Virtual DOM with the old one, node by node in a recursive manner, to figure out exactly where the changes have occurred. Based on these differences, it then decides the minimal number of operations required to update the Real DOM.

React takes two assumptions while performing this diffing:

1. Two elements of different types will produce different trees.

2. The developer can hint at which child elements may be stable across different renders with a key prop.

When comparing two trees, if React finds elements of different types, it simply destroys the old tree and builds a new one.

When comparing two same-type elements, the attributes are checked. If the same element but different attributes are found, React only updates the changed attributes.

For lists of children, React uses keys to identify items. If an old component has a key that is also present in the new component list, the component is moved instead of being destroyed and recreated, thereby maintaining its state.

What is this a good answer?

• Technical understanding: The answer demonstrates an understanding of the technical aspects of the diffing algorithm, including how it works and what it does.

• Detailed explanation: The response provides a detailed explanation, including the assumptions React makes during diffing, how differences are determined, and how updates to the DOM are decided.

• Contextual knowledge: The answer shows a practical understanding of how the algorithm affects performance in a real-world application, as well as how it's used in the process of reconciling the Virtual DOM with the Real DOM.

• Application of the concept: The response shows an ability to apply the knowledge of the diffing algorithm to optimize rendering, improve application performance, and maintain state in lists of components.

• Clarity and structure: The answer is clear and structured, explaining the process step by step, and making it easy for anyone (including non-technical interviewers or readers) to understand.

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How would you optimize a React application for performance?

Why is this question asked?

Optimizing applications is a fundamental skill for any React developer. The interviewer wants to know how good you are at this and what are the methods that you’re aware of.

This question can also reveal a lot about your experience and approach to building applications. It shows whether you are proactive in optimizing the performance of their applications and whether you understand the impact of various React features and techniques on performance.

Example answer:

There are several key techniques I frequently use.

To begin with, I implement lazy loading of components with React.Lazy, which allows me to split my code into different bundles that are loaded only when they're needed. This significantly reduces the initial load time of the application.

Also, I use React's built-in PureComponent when I know that a component’s output is determined solely by its props and state. PureComponent performs a shallow comparison of the component's props and state, preventing unnecessary renders when the props and state do not effectively change.

Next, I leverage the shouldComponentUpdate lifecycle method to control component rendering. This allows me to decide whether a component should re-render based on changes in state or props, which can prevent unnecessary renders and improve performance.

I also utilize hooks like useMemo and useCallback to optimize function components. useMemo allows me to memoize expensive function calls, so they are only re-computed when necessary, while useCallback helps to prevent unnecessary renders by returning a memoized version of the callback function.

For large lists, I use React's Virtual DOM to only render items that are visible to the user. This technique, also known as windowing, can drastically reduce the number of DOM nodes that need to be created and updated, improving performance.

Last but not least, I monitor performance with React's Profiler API to understand what is happening in my application and to find bottlenecks.

What is this a good answer?

• Comprehensive knowledge: The response demonstrates a broad knowledge of different performance optimization techniques available in React, such as lazy loading, shouldComponentUpdate, useMemo, useCallback, and windowing.

• Practical approach: The answer shows an applied understanding of these techniques, explaining when and why each one is used.

• Performance monitoring: The response includes the use of the React Profiler API to monitor performance, showing an ongoing commitment to performance optimization.

• Understanding of React concepts: The answer indicates an understanding of essential React concepts, including components, props, state, and lifecycle methods, as well as more advanced features like hooks and the React Profiler API.

• Forward-thinking: The answer shows that the developer considers performance throughout the development process, rather than treating it as an afterthought.

• Clarity: The response is clear and well-structured, making it easy for the interviewer or reader to follow the developer's thinking and understand their approach to performance optimization.

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Can you explain the concept of Virtual DOM and its benefits over Real DOM?

Why is this question asked?

The Virtual DOM is one of the core concepts behind React's efficiency and performance.

It's an abstraction of the Real DOM and plays a pivotal role in how React updates the user interface of web applications. Understanding this concept provides insights into how React works under the hood and why it's highly performant even for complex applications.

This question is crucial as it tests the knowledge and understanding of this fundamental concept.

If you understand the Virtual DOM and how it operates, you're more likely to write efficient and performant React code. What’s more, a good understanding of the Virtual DOM also helps in debugging and optimizing a React application.

Example answer:

The Virtual DOM is one of the key factors that sets React apart from many other JavaScript libraries and frameworks. It's a lightweight, abstract representation of the Real DOM.

Each time a change occurs in a React application, whether it's a user interaction or data change, a new Virtual DOM tree is created. This new Virtual DOM tree is then compared to the previous one using React's diffing algorithm. This process is known as reconciliation.

What’s so clever about the Virtual DOM is this reconciliation process.

Instead of making changes directly to the Real DOM (which can be slow and expensive), React calculates the minimal number of operations needed to update the Real DOM to match the new Virtual DOM. This is done by creating a "diff" or a patch between the old and new Virtual DOM trees.

React then batches these changes and updates the Real DOM in a single operation. This approach minimizes direct interactions with the Real DOM, which are computationally expensive and slow down the application.

That is what makes updates much faster and more efficient, leading to smoother, more responsive user interfaces.

So, I think there are two main advantages of the Virtual DOM:

• It greatly improves application performance by minimizing costly direct interactions with the Real DOM.

• It abstracts away the complexity of direct DOM manipulation, making it easier for developers to build complex user interfaces.

What is this a good answer?

• Understanding of Core Concepts: The response shows a deep understanding of the concept of the Virtual DOM and its significance in React. The answer explains how the Virtual DOM works and why it's beneficial.

• Technical Depth: The answer goes into the technical details of how React uses the Virtual DOM to perform efficient updates, highlighting the reconciliation process and how a "diff" is created between the old and new Virtual DOM trees.

• Practical Impact: The response includes the practical implications of the Virtual DOM, such as faster updates and more responsive user interfaces.

• Comparison to Real DOM: The answer clearly compares and contrasts the Virtual DOM with the Real DOM, emphasizing why the Virtual DOM is a more efficient and developer-friendly approach.

• Clarity and Organization: The answer is well-structured, clear, and concise, making the complex concept of the Virtual DOM easier to understand.

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How does React work with server-side rendering (SSR) and what are its benefits?

Why is this question asked?

SSR is the process of rendering web pages on the server before they reach the browser, which can lead to improved performance, better search engine optimization (SEO), and an improved user experience, particularly for users with slow internet connections.

This question is important because it delves into one of the advanced techniques used in React for optimization. It's also a clear marker of an experienced React developer, as implementing SSR requires a good understanding of both front-end and back-end development.

Example answer:

Server-side rendering (SSR) with React is a technique where the server renders the initial HTML of the page, instead of leaving it all to the client's browser. This approach can result in significant performance and SEO benefits.

In a traditional React application, the browser downloads a minimal HTML page, the JavaScript bundle, and then React generates the HTML for the components.

However, with SSR, the server runs the React code first and sends the generated HTML of the page to the browser. The browser can then display the HTML while the JavaScript bundle is still downloading.

Once the JavaScript is loaded, React on the client side takes over and it works like a regular React application.

There are quite a few benefits to using SSR in a React application.

First, of course, is that it can improve performance. Since the server sends the rendered HTML to the browser, the user can see the page faster, even before all the JavaScript has loaded. This results in a faster time to first paint (TTFP) and a better-perceived loading speed.

Also, SSR can significantly improve SEO. Some search engine crawlers may not effectively index pages that require JavaScript to render content. Since SSR sends a fully rendered page to the client, the crawler can see all content on the initial load.

Finally, SSR can improve the user experience on slow networks. Since the server does the heavy lifting of rendering the page, users on slower networks can still see the content quickly.

What is this a good answer?

• Detailed Explanation: The answer provides a comprehensive explanation of what SSR is and how it works in the context of React. It also explains the difference between SSR and traditional client-side rendering in React.

• Benefit Analysis: The response discusses the benefits of using SSR, including improved performance, better SEO, and an enhanced user experience on slow networks. This shows an understanding of why SSR can be important in real-world applications.

• Technical Insights: The answer discusses how React works with SSR, indicating a deep understanding of React's capabilities.

• Practical Application: Mentioning libraries like Next.js shows familiarity with the practical application of SSR in real-world React development.

• Clarity: The response is clearly structured and easy to understand, making complex concepts accessible.

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How would you handle error boundaries in React?

Why is this question asked?

Error boundaries are crucial when you’re trying to build robust and reliable applications. They allow you to catch and handle exceptions in the component hierarchy, preventing entire application crashes when something goes wrong in just one of the components.

The interviewer wants to know if you understand error handling and if you’re aware of all the techniques to do it. The idea is to understand your ability to build resilient applications that can handle unexpected failures gracefully.

Example answer:

Error boundaries in React are a way to catch and handle errors that occur in the component lifecycle methods, constructors, render method, and other parts of a component's lifecycle.

They act as a sort of 'catch block' for components, catching JavaScript errors, logging them, and displaying a fallback UI instead of showing a broken component to the user.

An error boundary is a React component that implements either or both of the lifecycle methods `getDerivedStateFromError` and `componentDidCatch`.

`getDerivedStateFromError`, of course, is used to render a fallback UI after an error is thrown, updating the state so that the next render will show the fallback UI.

On the other hand, `componentDidCatch` is used to catch JavaScript errors in the component subtree, log the error information, and is executed after an error has been thrown by a descendant component. This method can be used to log the error information to an error reporting service.

One thing that’s important to understand here is that error boundaries only catch errors in the components below them in the tree. An error boundary can’t catch an error within itself.

In practice, I would typically implement error boundaries in my higher-level components, so that if an error occurs in a lower-level component, it doesn't break the entire application. I would also make use of a centralized error logging service to keep track of these errors for future debugging and resolution.

What is this a good answer?

• Comprehensive Understanding: The response demonstrates a thorough understanding of error boundaries in React, and how they help handle errors in a component's lifecycle.

• Clear Explanation: The answer provides a clear, succinct explanation of what error boundaries are and how they function, breaking down the two lifecycle methods involved in error boundaries.

• Strategic Approach: The developer's approach to placing error boundaries in higher-level components indicates a strategic understanding of how to effectively use error boundaries to maintain application stability.

• Practical Application: The mention of using a centralized error logging service for tracking and debugging shows an understanding of best practices when dealing with errors in real-world applications.

• Focus on User Experience: The emphasis on preventing application crashes and displaying a fallback UI in case of errors shows the developer’s focus on maintaining a good user experience even in failure scenarios.

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How do React hooks work under the hood?

Why is this question asked?

React Hooks are a feature introduced in React 16.8 that allow you to use state and other React features without writing a class.

They've since become a fundamental part of React and understanding how they work under the hood can be a mark of a seasoned React developer.

Having a deeper understanding of Hooks means you can use them more effectively and avoid potential pitfalls.

They also provide important insights into React's design philosophy and give developers a better understanding of the 'React way' of doing things.

Example answer:

React Hooks are functions that let you "hook into" React state and lifecycle features from function components. The two most common hooks you'll see are `useState` and `useEffect`.

Under the hood, React maintains a list of hooks used in your component in the same order they are called.

This is why the rules of Hooks dictate that they should always be called at the top level of your React function and not inside loops or conditions, to ensure that the order of hook calls remains consistent across multiple renders.

When a component renders, React keeps track of the current executing component and has a pointer to the current hook. When you call a Hook like `useState`, it's able to associate the state variable with the Hook in the current function component.

For ‘useState’, React allocates an object that holds the current state value and a dispatch function to update that state. When the dispatch function is called, React updates the value and re-renders the component.

`useEffect`, on the other hand, is a way to handle side effects in function components.

When you call `useEffect`, you’re telling React to remember your effect function and call it later after performing the DOM updates.

It's deferred until after the render is committed to the screen, unlike componentDidMount and componentDidUpdate, which run immediately after the render.

The clean-up function in `useEffect` is used for canceling subscriptions, timers, or to invalidate the setup done in the effect.

What is this a good answer?

• In-Depth Understanding: The answer shows a solid understanding of how Hooks work internally, detailing the process React follows to manage state and side effects with Hooks.

• Explanation of Core Hooks: By discussing `useState` and `useEffect` in detail, the answer demonstrates knowledge of the most commonly used Hooks and their inner workings.

• Adherence to Rules of Hooks: The answer correctly emphasizes the importance of the rules of Hooks, explaining why they must be called at the top level and not inside loops or conditions.

• Comparison with Class Lifecycle Methods: By comparing `useEffect` with lifecycle methods like `componentDidMount` and `componentDidUpdate`, the answer shows an understanding of how Hooks have changed the way side effects are managed in React.

• Reflects React Philosophy: The answer reflects the principles of React, emphasizing how Hooks allow for more declarative and consistent code.

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Explain how React Router works.

Why is this question asked?

Routing is a crucial part of any modern single-page application (SPA), and React Router is the standard routing library for React.

Understanding React Router is essential for developers building SPAs with complex user interfaces and multiple routes.

The interviewer wants to understand your knowledge of routing in React and how you use React Router to handle navigation and component rendering based on the current URL.

Example answer:

React Router is a declarative, component-based routing library for React applications.

It enables the navigation among views of various components in a React Application, maintaining the seamless user experience of a single-page web application with the help of HTML5 history API.

The key concept in React Router is dynamic routing.

Dynamic routing means routing happens as your app is rendering, not in a configuration or convention outside of your components.

This is in contrast with static routing used in server-side applications where routes are declared at server startup.

React Router provides a collection of navigational components like <Router>, <Route>, and <Switch>.

The <Router> component is a wrapper that keeps track of the current URL.

<Route> allows you to render components based on the location's pathname.

<Switch> is used to render only the first <Route> or <Redirect> that matches the location.

React Router also supports nested routing, meaning routes can be defined inside components.

This is useful for creating components that are unaware of the application's overall layout, leading to a more modular and manageable codebase.

Lastly, React Router provides hooks that can be used to navigate programmatically or access the state of the router.

What is this a good answer?

• Understanding of Core Concepts: The answer shows a strong understanding of the core concepts in React Router, like dynamic routing and nested routing.

• Clarity: The explanation of how the key components like <Router>, <Route>, and <Switch> function together to enable routing in a React application is clear and easy to understand.

• Contrasting with Alternative Approaches: By contrasting dynamic routing with static routing, the answer provides a deeper understanding of the philosophy behind React Router and its advantages.

• Recognition of Modularity: The acknowledgment of the modular nature of nested routes shows the developer's appreciation for maintainable and manageable code.

• Knowledge of React Router Hooks: The mention of React Router's hooks shows an awareness of more advanced features in the library.

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Can you explain how context and context providers work in React?

Why is this question asked?

Understanding the Context API and how context providers work is fundamental for building scalable and efficient React applications.

The question is asked to assess your understanding of state management at a global level and your ability to avoid "prop drilling", where props are passed down through intermediate components to where they are needed.

Example answer:

React's Context API is a feature that allows you to share some value or state with multiple components without explicitly passing it through props.

This can be super helpful in a large application where some data needs to be accessible by many components at different nesting levels.

To use the Context API, you start by creating a Context object with `React.createContext()`. This object contains two main components: a Provider and a Consumer.

The Context Provider is a React component that allows consuming components to subscribe to context changes.

It accepts a `value` prop to be passed to consuming components that are descendants of this Provider. One provider can be connected to multiple consumers. This `value` prop is often used to share state across components.

The Consumer, on the other hand, is what you use in any component that needs to consume the value from the context.

When React renders a component that subscribes to this Context object, it will read the current context value from the closest matching Provider above it in the tree.

If there is no Provider for this context above, it will default to the value passed in React.createContext().

The Context API is especially useful for passing down data to deeply nested components, global state management, and sharing configuration.

What is this a good answer?

• Understanding of Core Concepts: The answer shows a strong understanding of the purpose of the Context API and its main components — Provider and Consumer.

• Clarity: The explanation is clear and precise, providing an accurate description of how context and context providers work.

• Real-world Application: The mention of use-cases for the Context API, such as global state management and sharing configuration, demonstrates the developer's awareness of practical applications for this feature.

• Solution for Common Problem: The answer acknowledges a common problem in React - prop drilling - and presents Context API as a solution, showing the developer's problem-solving abilities.

• Understanding of React's Tree Structure: The mention of how a component finds its matching Provider in the tree structure shows a deeper understanding of React's rendering and component hierarchy.

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What has been the most complex React application you've worked on and what were the key challenges you faced?

Why is this question asked?

This is basically your chance to brag. It’s your chance to show off. It’s your chance to tell the interviewer just how good you are.

Interview questions, however thorough, don’t give interviewers the full picture. So, interviewers often allow candidates to fill in the blanks. The reason your interviewer is asking this question is to find out what you consider complex topics, your ability to face challenges, and to understand how you approach problems.

Example answer:

The most complex React application I've worked on was a large-scale customer relationship management (CRM) system for a multinational corporation.

The application was complex because it had multiple layers of components, several user roles with different permissions, real-time data sync, and a host of features like messaging, task management, document storage, and advanced analytics.

One of the key challenges I faced was managing the application's state, as it was a massive application with a lot of interconnected parts. To solve this, I used Redux for global state management and broke down the state into manageable chunks.

Another challenge was handling real-time data synchronization across different parts of the application.

We solved this by implementing WebSockets for real-time updates, but managing these connections and ensuring the UI stayed in sync with the server presented its own set of challenges.

Lastly, the CRM had to support different user roles, each with its own set of permissions and views. Managing these user roles and permissions was a complex task due to the deep nesting of components and routes in the application.

Despite these challenges, working on this application was a great learning experience. It taught me a lot about state management, real-time data handling, and effective component structuring. It also improved my skills in debugging, testing, and performance optimization.

What is this a good answer?

• Detailed Explanation: The developer provides a comprehensive overview of the CRM system, detailing its complexities and key features.

• Problem-solving Skills: The developer not only explains the challenges faced but also details the solutions implemented, showcasing their problem-solving skills.

• Technical Knowledge: The mention of Redux, WebSockets, and permission management demonstrates the developer's knowledge of a variety of tools and techniques.

• Experience with Complex Applications: The developer's experience with a large-scale, complex application is clear, and they provide insights into the challenges such projects often present.

• Growth and Learning: The developer emphasizes what they learned from the experience, showing an ability to grow and learn from challenging situations.

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Can you share an experience where a deeper understanding of React led to a significant performance improvement or problem solution?

Why is this question asked?

This is where you get to show off how experienced you are and how much knowledge you really have. It’s an invitation to demonstrate your understanding of the internal workings of React.

It also helps the interviewer gauge your analytical thinking, problem-solving skills, and your ability to optimize React applications for better performance.

Example answer:

One project immediately comes to mind. It was a large e-commerce store and the application was facing performance issues, especially when loading the product listing page with thousands of products. The initial load time was quite high and the UI was noticeably laggy when scrolling.

On investigating the issue, I realized that the problem was related to unnecessary re-renders of the product components.

Every product component was re-rendering whenever there was a state change anywhere in the product list, regardless of whether it affected that particular product or not.

To solve this problem, I used React's `React.memo()` function.

React.memo(), of course, is a higher-order component that memorizes the rendered output of your component and then re-uses it the next time it renders, as long as the props haven't changed.

I wrapped the product component with React.memo(), which meant that the component would only re-render if the props it cared about had changed.

The result was a significant improvement in the page's performance. The initial load time reduced noticeably, and the UI no longer lagged when scrolling.

Not only did this enhance the user experience, but it also taught me a valuable lesson about the importance of understanding how React works under the hood and how this knowledge can be applied for performance optimization.

What is this a good answer?

• Practical Application: The developer provides a real-world example of a problem they faced and how they used their React knowledge to solve it, illustrating their ability to apply theoretical knowledge to practical problems.

• Problem-Solving Skills: The developer effectively identified the issue and used an advanced feature of React to solve the problem, demonstrating strong problem-solving skills.

• Performance Improvement: The developer made a significant improvement in the application's performance, which directly impacts user experience, showcasing the importance of understanding performance optimization in React applications.

• Learning from Experience: The developer reflects on what they learned from the experience

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Conclusion:

There you are — 10 important advanced React interview questions and answers. The concepts covered in these questions will make up a huge part of your React technical interview. In fact, a lot of smaller, simpler questions are answered within the blog.

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Visit Simple Job Listings and find amazing remote front-end developer jobs. Good luck!