Top 40 ReactJS Interview Questions and Answers for 2023/

As ReactJS continues to dominate the web development space, hiring the right talent with expertise in this library is paramount for businesses looking to build scalable and robust web applications. To help you screen the best React developers in 2023, we have compiled a list of the most asked ReactJS interview questions and their answers.

Most Asked ReactJS Interview Questions

1. What is ReactJS and why would you use it?
ReactJS is a JavaScript library developed by Facebook to build user interfaces, especially for single-page applications. It allows developers to create reusable UI components, which is beneficial in maintaining consistency across the application. One of the standout features of React is its virtual DOM implementation, which significantly improves the performance by minimizing direct manipulations of the actual DOM and optimizing re-renders. React’s component-based architecture also makes it easier to manage and scale applications, as components can be developed and tested independently.

2. Describe JSX.
JSX, or JavaScript XML, is a syntax extension for JavaScript. It’s recommended by React to describe the UI of the application. It resembles HTML, allowing developers to write React components using a syntax they are familiar with. However, JSX is not mandatory for React development but offers a more visual and concise way to describe the UI within the JavaScript code. It gets transcompiled by tools like Babel into standard JavaScript functions that browsers can understand.

3. Explain the difference between a class component and a functional component.
Class components are ES6 classes that extend from React.Component. They can hold state, have lifecycle methods, and can be reused. Functional components, on the other hand, are simpler JavaScript functions that return JSX. Initially, functional components were stateless, but with the introduction of React Hooks, they can now manage state and access lifecycle methods, providing nearly all the capabilities of class components but with a more concise and readable syntax.

4. What are Props in React?
Props (short for “properties”) are a mechanism for passing data from parent to child components in React. They are read-only, ensuring that child components cannot modify the data they receive. This adheres to the “unidirectional data flow” philosophy of React, where the state is passed down from parent components as props to child components.

5. What is a state in React and how is it used?
The state represents any data that can change over time and affect the component’s rendering. In React, each component has its own state and changes to this state cause the component to re-render. It provides a mechanism to store dynamic data and allows components to become interactive. The state is initialized in the constructor of class components and can be modified using the setState method. With hooks in functional components, the state can be managed using the useState hook.

6. Explain the component lifecycle in React.
In React, class components have lifecycle methods that execute in specific phases of a component’s life: mounting, updating, and unmounting. Some key lifecycle methods are componentDidMount, which is called after a component is added to the DOM; componentDidUpdate, called after a component updates; and componentWillUnmount, called just before a component is removed from the DOM. These methods provide opportunities to perform actions based on changes in the component’s lifecycle.

7. What are React Hooks?
React Hooks are a feature introduced in React 16.8 that allows developers to use state and other React features in functional components. Prior to hooks, these features were exclusive to class components. Common hooks include useState for managing state, useEffect for side effects, and useContext for accessing context. Hooks enable developers to write cleaner and more readable code by using functional components instead of classes.

8. How do you handle forms in React?
In React, forms maintain their own internal state. To handle form input, developers use controlled components, where the form field’s value is controlled by the state of the component. Every change to the input updates the state, and the value is set using the value attribute. Functions are used as event handlers to capture changes and update the component’s state accordingly.

9. What is the Context API in React?
The Context API provides a way to share values like themes, authentication status, and other data between components without passing props down manually at every level. It makes it easier to manage global data in an application, like user authentication or theme settings. With the Context API, a value is provided at the top level and can be consumed by any descendant, no matter how deep they are in the component tree.

10. How does React’s virtual DOM work?
React’s virtual DOM is a lightweight representation of the actual DOM. Whenever a change occurs in a React application, a new virtual DOM is created and compared to the previous one using a ‘diffing’ algorithm. This identifies the minimum number of changes required. Only those changes are then updated in the real DOM, resulting in optimized and efficient updates, improving performance, and ensuring smoother user experiences.

11. What are the keys in React and why are they important?
Keys are special string attributes that help React identify which items in a list have changed, been added, or been removed. They should be unique among siblings. Using keys ensures efficient updates and re-renders of lists by minimizing component instance and DOM node creation and destruction, thereby improving performance.

12. What are refs in React?
Refs provide a way to access the DOM nodes or React elements created in the render method. They can be particularly useful in scenarios where you need to measure or focus DOM nodes. While generally advised to use props and state for data management, refs are useful for actions that require direct access, like triggering animations or integrating with third-party DOM libraries.

13. What is Redux and how does it relate to React?
Redux is a predictable state container for JavaScript apps, often used with React. It provides a centralized store for the state that needs to be used across the entire application, with rules ensuring that the state is always kept consistent. In conjunction with React, Redux ensures that state changes in the store trigger re-renders in relevant components.

14. Explain the difference between controlled and uncontrolled components.
Controlled components have their form data controlled by the React component state. Their values and changes are controlled using functions setState and properties like value, ensuring the component always displays the current value. Uncontrolled components, on the other hand, store their data in the DOM itself. They are more similar to traditional HTML form inputs and are accessed using refs.

15. How is React different from other JavaScript frameworks?
React is primarily focused on building user interfaces, emphasizing a component-based architecture, making UI elements reusable and maintainable. While it can be integrated into various backend technologies, React itself doesn’t dictate any specific technology stack. Its virtual DOM implementation provides optimized updates and rendering. Unlike full-fledged frameworks like Angular, React is more flexible, allowing developers to integrate with various libraries based on their requirements.

16. What is the significance of React keys in iterators?
Keys are crucial in iterators because they help React identify changes in a list’s elements. This aids in determining which items need to be re-rendered or maintained, resulting in optimized performance. Without unique keys, React might face difficulties in determining which child components need updating, leading to inefficient updates and potential issues in the component state.

17. What is the significance of arrow functions in React?
Arrow functions play a vital role in React due to their behavior with the this keyword. In JavaScript, the context this can change based on how a function is called. However, arrow functions don’t have their own this context, so they inherit the this value from the surrounding scope. This is especially useful in React event handlers, where using arrow functions ensures that this always refers to the component instance.

18. What is the purpose of React Router?
React Router provides a way to add navigation into React applications. It enables the definition of multiple routes and rendering components based on the browser’s URL. This ensures that users can navigate between different parts of the application and even utilize browser navigation features like the back button, all while maintaining the single-page application feel.

19. How can you optimize React performance?
React’s performance can be optimized in various ways: using the virtual DOM efficiently by minimizing state changes and ensuring component re-renders only when necessary; using tools like React.PureComponent and React.memo to avoid unnecessary renders; optimizing child components with shouldComponentUpdate and splitting the application code using lazy loading and dynamic imports. Additionally, using tools like the React DevTools profiler can help identify bottlenecks.

20. How do you manage state in large React applications?
For large React applications, the local component state might not suffice. Tools like Redux or Context API provide centralized state management. Redux, for instance, maintains a single store for the entire app, ensuring a consistent state across the application. Actions and reducers specify how state changes and middleware can be used for side effects. The Context API can be used for passing down states without prop drilling, especially useful for global states like themes or user authentication.

21. What is the significance of keys in React lists? Keys in React provide a mechanism for uniquely identifying elements within lists or arrays. They play a crucial role in optimizing React’s rendering process. When elements in a list change — due to additions, deletions, or reordering — React uses keys to determine which items have changed, and updates only those items. Without specific keys, React might re-render more elements than necessary, causing a performance hit. Hence, using keys effectively results in a more efficient and faster update process in the DOM.

22. What is a higher-order component (HOC)? Higher-order components, or HOCs, are a foundational concept in React that revolves around the principle of composition. At its core, a HOC is a function that takes a component and returns a new component with additional props or functionalities. They are instrumental in code reusability, allowing developers to share logic across components. Think of them as pure JavaScript functions, aiming to adhere to the “Don’t Repeat Yourself” (DRY) principle in coding, making React applications cleaner and more efficient.

23. How can you handle forms in React? Handling forms in React is commonly done using controlled components. In this paradigm, the form elements’ data — such as the value of an input field — is stored in the component’s state. When a user interacts with the form, for example by typing into an input field, a function is called to update the state with the new value. This way, React remains the single source of truth for the form data. By harnessing the power of React’s state and the onChange event, developers have granular control over form validation, submission, and other interactions.

24. What is the difference between a controlled and an uncontrolled component? Controlled components in React have their value managed by the component’s state. Every change to the input updates the state, and the component re-renders with the new value. This ensures that the displayed value and the state always sync up. On the other hand, uncontrolled components store their value in the DOM itself, similar to traditional HTML form elements. With uncontrolled components, React does not manage the element’s value, and you query the DOM using refs to access its value. In general, controlled components offer more flexibility and are recommended for most situations.

25. How do you handle asynchronous operations in Redux? Asynchronous operations, such as API calls or delayed actions, in Redux, are commonly managed using middleware. Redux Thunk is one such middleware that allows you to write action creators that return a function instead of an action, enabling asynchronous operations. Another popular middleware, Redux-Saga, uses generator functions to handle side effects and makes asynchronous flows more readable and easier to manage. By using these middlewares, developers can maintain the principles of Redux while seamlessly handling asynchronous tasks.

26. What is a ref in React? In React, “ref” stands for reference and provides a way to access the properties of a DOM element or a class component directly. It bypasses the typical data flow mechanisms of React, like props or state, offering a more imperative approach. Using refs, developers can focus on specific DOM elements, trigger animations, manage focus, or integrate with third-party libraries. However, while powerful, refs should be used sparingly and only when necessary, as they can make the component logic harder to follow.

27. What are PropTypes in React? PropTypes is a mechanism in React that allows developers to specify the types of data a component should receive as props. It serves as a form of runtime type checking and documentation. By setting PropTypes, developers establish clear expectations for the kind of data a component should receive, which can prevent potential bugs and clarify component usage. When a component receives props that don’t match the specified PropTypes, React will produce a warning in the console, helping developers catch issues during development.

28. Why is the shouldComponentUpdate lifecycle method useful? The shouldComponentUpdate lifecycle method is a performance optimization tool in React. It’s called right before the re-rendering process and allows a component to prevent the render based on specific conditions. By comparing the current props and state with the next ones, developers can determine if re-rendering is genuinely necessary. If the method returns false, the component won’t re-render, saving processing time and boosting app performance. This selective rendering becomes crucial in applications with frequent state changes or large component trees.

29. How can you handle exceptions in React components? Error handling in React components is facilitated using error boundaries. An error boundary is a React component that captures JavaScript errors anywhere in its child component tree, logs those errors, and displays a fallback UI instead of the component tree that crashed. By wrapping parts of the application in error boundary components, developers can prevent crashes from corrupting the entire app, providing a more resilient user experience. It’s worth noting that error boundaries catch errors during rendering, in lifecycle methods, and in constructors, but not in event handlers or asynchronous code.

30. What is the role of the React.Fragment? React.Fragment is a lightweight wrapper that allows developers to return multiple elements from a component’s render method without adding extra nodes to the DOM. Prior to its introduction, wrapping multiple elements required adding a surrounding div or another container, which could result in unnecessary parent divs in the DOM and potential styling or layout issues. React.Fragment solves this by grouping elements without introducing any additional markup, making the rendered output cleaner and more semantically accurate.

31. How can you integrate React with other popular technologies and platforms like WordPress? Integrating React with platforms like WordPress can be achieved through several means. One approach involves using frameworks like “React-WordPress-scripts” or “Frontity”, which facilitate the creation of React-based themes for WordPress. By harnessing the REST API provided by WordPress, React can fetch and display content seamlessly. This hybrid approach combines the content management capabilities of WordPress with the interactive and dynamic nature of React, resulting in powerful, modern web applications.

32. What is the React virtual DOM? React’s virtual DOM is a programming concept where an in-memory representation of the real DOM elements is kept. The rendering engine can quickly make changes to the virtual DOM and subsequently update the real DOM in a more optimized and efficient manner. When a user interacts with a React app, and changes are made, React first updates the virtual DOM using a process called reconciliation. Then, using a ‘diffing’ algorithm, it compares the current virtual DOM with the previous one, calculating the most efficient way to update the actual DOM, leading to better performance and user experience.

33. How does React handle events? React wraps native browser events into synthetic events to ensure consistent properties and behaviors across different browsers. This wrapping provides a cross-browser interface to the native events, meaning that the developer doesn’t have to worry about browser-specific quirks when working with events. Synthetic events in React have the same interface as native events but benefit from React’s event delegation mechanism. This results in improved performance and a more consistent event-handling system.

34. Can you explain the concept of a stateless component? A stateless component in React is a component that does not manage or use an internal state. Instead, it relies entirely on props passed down from its parent component for rendering. Stateless components, often referred to as functional components (especially before the introduction of hooks), are simpler and more predictable since they render the same output for the same props. They’re generally easier to maintain and test, making them a preferred choice for presentational parts of an application where side-effects and lifecycle methods aren’t required.

35. What is React Router and its primary use? React Router is a powerful routing library built on top of React, which helps in adding navigation to the application. It enables navigation among different views and ensures that the user interface is synchronized with the URL. React Router provides components, as <Route> and <Link>, that facilitate the declaration of routes and the linking between views. By enabling dynamic route matching and seamless transitions between views, React Router plays a pivotal role in building single-page applications (SPAs) that feel like multi-page ones.

36. How do you optimize performance in React apps? Performance optimization in React apps can be approached in various ways. Firstly, the shouldComponentUpdate lifecycle method or using PureComponent can prevent unnecessary re-renders. Techniques like memoization can ensure that computations are not repeated unnecessarily. Virtualizing long lists with tools like react-window this can ensure only visible items are rendered. Code splitting with React.lazy() and Suspense can reduce the initial load time. Additionally, the built-in React Developer Tools provides profiling capabilities to pinpoint performance bottlenecks and optimize them.

37. What are React Portals? React Portals provide a first-class way to render child components into a DOM node outside of the parent DOM hierarchy while retaining the React context. This is especially useful for components like modals, pop-ups, or tooltips. Even though a portal can be anywhere in the DOM tree, it behaves like a normal React child in every other way. Features like context work identically, ensuring a consistent development experience. Portals empower developers to break out of the DOM tree without losing the benefits of React’s reactivity.

38. What is the significance of the render prop pattern? The render prop pattern in React revolves around a component accepting a prop, usually named render, which is a function. This function returns React elements, allowing consumers of the component to inject content. The pattern is a way to share code between components using a prop whose value is a function. It promotes component reusability without altering its structure. By inverting the control to the caller, the render prop pattern provides greater flexibility in rendering whatever is needed, be it a list, a modal, or any other UI element.

39. How is React different from Angular? React and Angular, while both powerful front-end frameworks have distinct differences. React, developed by Facebook, is a library focused primarily on building user interfaces. It emphasizes component-based architecture, and developers often pair it with other libraries to create a full-fledged application. On the other hand, Angular, developed by Google, is a comprehensive framework providing a wide array of tools and features out of the box, such as dependency injection, two-way data binding, and an extensive suite of testing tools. The choice between React and Angular depends on specific project requirements, developer preference, and the desired ecosystem.

40. What is Fragment in React?
A Fragment in React is a common pattern for a group of elements without adding extra nodes to the DOM. Sometimes, you might want to return multiple elements from a component. Wrapping these elements in a parent div might not be ideal due to styling or layout concerns. Fragments let you group a list of children without adding unnecessary nodes to the DOM, resulting in a cleaner and more semantic output.

Conclusion:

React has firmly established itself as a frontrunner in the landscape of UI libraries and frameworks. Its component-based architecture, coupled with the power of the virtual DOM, offers developers a robust platform to build interactive and highly efficient web applications. As we navigated through these interview questions, it’s evident that React’s ecosystem, ranging from hooks to the Context API and integrations with state management solutions like Redux, offers comprehensive tools to address varying development needs.

Moreover, the evolution of React has been synonymous with the changing paradigms of web development. The introduction of hooks, for instance, highlights React’s commitment to functional programming, allowing developers to harness state and lifecycle features without diving into class components. Similarly, the Context API and React Router demonstrate the library’s intention to create a holistic development environment, reducing the need for third-party libraries.

React’s compatibility with various backend technologies makes it a versatile choice, fitting seamlessly into diverse tech stacks. Furthermore, with the continuous support from Facebook and a thriving community, React is not only relevant but continues to be a pivotal player in modern web development. Whether you’re a hiring manager, a budding React developer, or a seasoned professional looking to brush up your knowledge, understanding these nuances of React will undoubtedly serve you well in the dynamic world of web development.

To stay informed about the latest developments in React, visit slashdev.io. Our platform offers valuable insights, tutorials, and resources to help you make informed decisions in your web development journey.