Top 30 Most Common Interview Questions On React Js You Should Prepare For
Written by
James Miller, Career Coach
Landing a developer role often hinges on demonstrating your expertise in core technologies. For front-end positions, proficiency in React.js is frequently a key requirement. As React continues to dominate the landscape of user interface development, facing interview questions on React.js is inevitable. These interviews aren't just about recalling syntax; they test your understanding of fundamental concepts, your ability to build efficient and maintainable applications, and your grasp of best practices. Preparing thoroughly for common interview questions on React.js can significantly boost your confidence and performance. This guide provides a comprehensive look at 30 frequently asked questions, offering insights into why they are asked and how to provide clear, impactful answers. Mastering these questions will showcase your solid foundation and readiness to tackle real-world React development challenges, setting you apart from other candidates.
What Are interview questions on react js?
Interview questions on React.js cover a wide spectrum of topics designed to assess a candidate's knowledge and experience with the library. These questions typically range from fundamental concepts like JSX, components, state, and props to more advanced areas such as Hooks, Context API, reconciliation, performance optimization, and error handling. They often delve into practical aspects like handling forms, routing, and integrating with state management libraries. Beyond syntax and APIs, effective interview questions on React.js aim to understand your problem-solving approach, how you structure applications, and your awareness of performance considerations. Preparing for interview questions on React.js means not just memorizing definitions but understanding the underlying principles and trade-offs involved in building robust and scalable React applications.
Why Do Interviewers Ask interview questions on react js?
Interviewers ask interview questions on React.js for several crucial reasons. Firstly, they need to verify that candidates possess the necessary technical skills and understanding to work effectively with the library. Questions about core concepts confirm foundational knowledge, while those on Hooks or Context gauge familiarity with modern practices. Secondly, interviewers use these questions to evaluate a candidate's problem-solving abilities and how they approach common challenges in React development, such as state management or performance issues. Discussing these topics reveals logical thinking and practical experience. Finally, interview questions on React.js help determine cultural fit and communication skills; a candidate who can clearly explain complex topics and discuss design choices demonstrates valuable teamwork attributes. Preparing for common interview questions on React.js is essential to make a strong impression.
Preview List
What is React?
What are the advantages of using React?
What is JSX?
What are components in React?
What is the difference between a React element and a component?
What is the Virtual DOM?
What are keys in React and why are they important?
What is the difference between state and props?
What is the lifecycle of a React component?
What are React Hooks?
What is one-way data binding?
What is a higher-order component (HOC)?
Explain React Fragments.
What is Context API?
Difference between controlled and uncontrolled components in React?
What is reconciliation?
How do you optimize React performance?
What is React Router?
What are error boundaries?
What is the difference between function and class components?
What is event handling in React?
What is prop drilling and how to avoid it?
How do you handle forms in React?
What are React portals?
What is the significance of PureComponent?
What is server-side rendering (SSR) in React?
What is the difference between React and Angular?
How does React handle forms validation?
What is the role of Redux with React?
What are lazy loading and code splitting?
1. What is React?
Why you might get asked this:
This is a foundational question to check if you know what React is at a high level before diving deeper. It tests your basic understanding of its purpose.
How to answer:
Define React, its primary use case (UI library), and key characteristics like component-based architecture and the Virtual DOM.
Example answer:
React is a JavaScript library for building user interfaces, primarily single-page applications. It lets you create reusable UI components and manages updates efficiently using a virtual DOM, focusing on the view layer of applications.
2. What are the advantages of using React?
Why you might get asked this:
Interviewers want to see if you understand the benefits React brings to development, justifying its popularity and use.
How to answer:
Mention key benefits like the Virtual DOM for performance, component reusability, strong community support, and suitability for building complex UIs.
Example answer:
Advantages include improved performance via the Virtual DOM, component reusability reducing development time, a large ecosystem, unidirectional data flow for stability, and ease of integration with other libraries or frameworks.
3. What is JSX?
Why you might get asked this:
JSX is fundamental to writing React code. Understanding it is essential for working with React components effectively.
How to answer:
Explain that JSX is a syntax extension for JavaScript that allows writing HTML-like structures directly within JavaScript code. Mention it must be transpiled.
Example answer:
JSX stands for JavaScript XML. It's a syntax extension used with React to describe UI structure within JavaScript files. It looks like HTML but is actually JavaScript and gets compiled by tools like Babel into standard JS calls.
4. What are components in React?
Why you might get asked this:
Components are the building blocks of React applications. This question assesses your understanding of their role and nature.
How to answer:
Describe components as independent, reusable UI pieces. Mention they can be functional or class-based and receive props to render elements.
Example answer:
Components are the core building blocks of React applications. They are independent, reusable pieces of code that define how a part of the UI looks and behaves. They accept inputs (props) and return React elements to render.
5. What is the difference between a React element and a component?
Why you might get asked this:
This question tests your precise understanding of React's internal workings and terminology, which is crucial for debugging and optimization.
How to answer:
Define Element as a plain object describing what to render (immutable) and Component as the template (function or class) that returns elements (reusable).
Example answer:
A React element is a plain object representing a DOM node or another component. It's immutable. A component is a function or class that can accept props and state and returns a React element tree to be rendered.
6. What is the Virtual DOM?
Why you might get asked this:
Understanding the Virtual DOM is key to grasping how React achieves performance optimizations and updates the UI efficiently.
How to answer:
Explain it's an in-memory representation of the real DOM. Describe the diffing process: React builds a new VDOM, compares it to the old one, and updates only the changes in the real DOM.
Example answer:
The Virtual DOM is a lightweight copy of the actual browser DOM kept in memory. React uses it to compare the new UI state with the previous one (diffing) and efficiently update only the necessary parts of the real DOM.
7. What are keys in React and why are they important?
Why you might get asked this:
Keys are critical for list rendering performance and correctness. Misunderstanding keys leads to bugs and inefficiency.
How to answer:
Explain keys as unique identifiers for elements in lists. State their importance for React's reconciliation algorithm to efficiently identify changes (additions, removals, updates).
Example answer:
Keys are unique identifiers for items when rendering lists in React. They help React efficiently update the UI by allowing it to track which items have changed, added, or removed, improving performance and preventing potential bugs with element state.
8. What is the difference between state and props?
Why you might get asked this:
This is a fundamental distinction in React for managing data flow and component behavior.
How to answer:
Define Props as read-only inputs passed from parent to child, immutable by the child. Define State as mutable data managed within a component, used for dynamic behavior.
Example answer:
Props (properties) are data passed from a parent component to a child component; they are immutable within the child. State is data managed internally by a component, which can change over time and trigger re-renders.
9. What is the lifecycle of a React component?
Why you might get asked this:
Understanding the component lifecycle is essential for executing code at specific points, such as fetching data after mounting or cleaning up before unmounting.
How to answer:
Describe the main phases: Mounting (component is created and inserted), Updating (component re-renders due to state/prop changes), and Unmounting (component is removed). Mention associated methods/hooks.
Example answer:
React components go through phases: Mounting (initial rendering), Updating (re-renders), and Unmounting (removal). Key moments include just before render, after render (componentDidMount/useEffect), and before removal (componentWillUnmount/useEffect cleanup).
10. What are React Hooks?
Why you might get asked this:
Hooks are a major feature introduced in React 16.8. Knowledge of Hooks shows familiarity with modern React development practices.
How to answer:
Define Hooks as functions that let you "hook into" React state and lifecycle features from functional components. Mention common ones like useState and useEffect.
Example answer:
Hooks are functions introduced in React 16.8 that enable functional components to use state, lifecycle methods, and other React features previously only available in class components, like useState and useEffect.
11. What is one-way data binding?
Why you might get asked this:
React's unidirectional data flow is a core principle affecting how data is managed and debugged.
How to answer:
Explain that data flows only in one direction in React – typically from parent components down to child components via props. This makes the application state predictable.
Example answer:
One-way data binding means data in React flows in a single direction, usually downwards from parent components to children via props. This makes state management and debugging easier and more predictable compared to two-way binding.
12. What is a higher-order component (HOC)?
Why you might get asked this:
HOCs are a pattern for code reuse and logic abstraction, demonstrating understanding of advanced React patterns.
How to answer:
Define an HOC as a function that takes a component as input and returns a new component with enhanced functionality or props. Give examples of use cases like data fetching or state management.
Example answer:
A Higher-Order Component (HOC) is a function that accepts a component and returns a new, enhanced component. It's a pattern for reusing component logic, like connect from React-Redux or withRouter from React Router.
13. Explain React Fragments.
Why you might get asked this:
Fragments are a useful feature for avoiding unnecessary DOM nodes, showing attention to detail and performance.
How to answer:
Explain that Fragments allow grouping a list of children without adding extra nodes to the DOM (unlike wrapping in a div). Mention their short syntax (<>...).
Example answer:
React Fragments let you group multiple elements returned by a component without adding a wrapper DOM node like a div. This helps keep the DOM structure flatter and cleaner. They can be written as ... or the shorthand <>....
14. What is Context API?
Why you might get asked this:
Context API is React's built-in solution for managing shared state without prop drilling, relevant for modern React architecture.
How to answer:
Describe Context API as a way to pass data through the component tree without manually passing props down at every level. Explain its use for global state like themes or user authentication.
Example answer:
The Context API provides a way to share data, like theme or user authentication state, throughout your component tree without having to pass props down manually through nested components (prop drilling).
15. Difference between controlled and uncontrolled components in React?
Why you might get asked this:
This distinction is fundamental to handling form inputs correctly in React applications.
How to answer:
Define Controlled Components as those where form data is managed by React state. Define Uncontrolled Components as those where form data is managed by the DOM itself, typically accessed via refs.
Example answer:
In controlled components, form element data is handled by the component's state, and state updates with every input change. In uncontrolled components, form data is handled by the DOM directly, often accessed using refs when submitting the form.
16. What is reconciliation?
Why you might get asked this:
Reconciliation is the process behind React's efficient updates, demonstrating a deeper understanding of its core mechanism.
How to answer:
Explain reconciliation as the process React uses to update the browser's DOM. It involves comparing the new Virtual DOM tree with the old one (diffing) and applying only the necessary changes to the real DOM.
Example answer:
Reconciliation is the algorithm React uses to update the UI efficiently. When state or props change, React creates a new virtual DOM tree, compares it to the previous one (diffing), and calculates the minimal changes needed to update the actual DOM.
17. How do you optimize React performance?
Why you might get asked this:
Performance is crucial for user experience. This question tests your awareness of common optimization techniques.
How to answer:
List several techniques: using React.memo (for function components) or PureComponent (for class components), useCallback and useMemo hooks, keys in lists, code-splitting (lazy loading), and avoiding unnecessary re-renders.
Example answer:
Optimize using React.memo or PureComponent to prevent unnecessary re-renders, useCallback and useMemo for memoizing functions and values, providing stable keys in lists, using code-splitting, and leveraging the React DevTools profiler.
18. What is React Router?
Why you might get asked this:
Most single-page applications need routing. Knowledge of React Router (the de facto standard) is often required.
How to answer:
Identify React Router as a third-party library used for declarative routing in React applications. Mention its role in handling navigation between different components/views based on URL changes.
Example answer:
React Router is a standard library for handling navigation and routing in React applications. It allows defining different views (components) that should be displayed based on the URL, enabling single-page application navigation.
19. What are error boundaries?
Why you might get asked this:
Error handling is vital for creating robust applications. Error boundaries provide a structured way to manage errors in the UI.
How to answer:
Define Error Boundaries as React components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of crashing the whole application.
Example answer:
Error boundaries are components that can catch JavaScript errors in their child component tree, log them, and display a fallback UI. They prevent the entire application from crashing due to an error in one part of the UI.
20. What is the difference between function and class components?
Why you might get asked this:
This tests your understanding of React's evolution and the modern preference for function components with Hooks.
How to answer:
Explain that Function Components are simpler, stateless (before Hooks), and preferred now. Class Components can hold state and use lifecycle methods but are more verbose.
Example answer:
Historically, function components were stateless, while class components could manage state and lifecycle. With Hooks (React 16.8+), function components can also use state and lifecycle features (useState, useEffect), making them the modern standard due to their simplicity.
21. What is event handling in React?
Why you might get asked this:
Handling user interactions is a core part of building UIs. This checks your familiarity with React's event system.
How to answer:
Explain that React uses a synthetic event system, which wraps native browser events to provide a consistent cross-browser API. Mention using camelCase for event names (onClick instead of onclick) and passing functions as event handlers.
Example answer:
React handles events using a synthetic event system, which provides a cross-browser wrapper around native events. Event names are camelCase (e.g., onClick), and you pass a function reference as the handler. event.preventDefault() works as expected.
22. What is prop drilling and how to avoid it?
Why you might get asked this:
Prop drilling is a common issue in component hierarchies. Knowing how to avoid it shows good design practices.
How to answer:
Define prop drilling as passing data through several layers of nested components when only a deeply nested child component needs it. Suggest using Context API or state management libraries (like Redux, Zustand, Recoil) to avoid it.
Example answer:
Prop drilling is passing data down through multiple components in a hierarchy simply because a grandchild or deeper component needs it. You can avoid it by using React's Context API or a dedicated state management library to provide data directly where needed.
23. How do you handle forms in React?
Why you might get asked this:
Forms are ubiquitous in web applications. This checks your practical ability to manage user input.
How to answer:
Describe using controlled components where form input values are tied to component state and updated via onChange handlers. Mention using uncontrolled components with useRef for simpler forms.
Example answer:
Forms are typically handled using controlled components, where the input's value is managed by React state. An onChange handler updates the state, and the input's value prop is set from the state. Uncontrolled components use refs to access form values directly.
24. What are React portals?
Why you might get asked this:
Portals are useful for specific UI patterns (modals, tooltips) where a component needs to render outside its parent's DOM hierarchy.
How to answer:
Explain that Portals provide a way to render a child component into a DOM node that exists outside the DOM hierarchy of the parent component. Mention their use for modals, dialogs, and tooltips.
Example answer:
React Portals allow rendering children into a different part of the DOM tree, outside the parent component's hierarchy. This is useful for components like modals, dialogs, or tooltips that need to appear on top of other elements but are logically tied to a component lower in the tree.
25. What is the significance of PureComponent?
Why you might get asked this:
PureComponent is a performance optimization tool for class components, showing knowledge of older but still relevant techniques.
How to answer:
Explain that PureComponent is similar to Component but implements shouldComponentUpdate with a shallow comparison of props and state. It automatically prevents re-renders if props/state haven't shallowly changed.
Example answer:
React.PureComponent is a class component that automatically implements shouldComponentUpdate with a shallow comparison of its props and state. This prevents unnecessary re-renders if the shallowly compared props and state are unchanged, improving performance.
26. What is server-side rendering (SSR) in React?
Why you might get asked this:
SSR is important for performance (initial load time) and SEO, demonstrating awareness of production concerns.
How to answer:
Define SSR as rendering React components on the server and sending the static HTML to the client. Explain benefits like faster initial page load and improved search engine indexing.
Example answer:
Server-side rendering (SSR) is rendering your React application on the server instead of the client. The server sends the initial HTML to the browser, improving initial load time and making the content crawlable by search engines before the JavaScript loads.
27. What is the difference between React and Angular?
Why you might get asked this:
This comparison evaluates your understanding of React's place in the front-end ecosystem relative to other major frameworks.
How to answer:
Highlight key differences: React is a UI library (view layer focus) while Angular is a full framework (more opinionated, includes routing, HTTP client, etc.). Mention their different approaches to the DOM (Virtual DOM vs. real DOM) and data binding (unidirectional vs. two-way).
Example answer:
React is a JavaScript library focused only on building user interfaces using a Virtual DOM and unidirectional data flow. Angular is a comprehensive framework with built-in features like routing and HTTP, using two-way data binding and the real DOM.
28. How does React handle forms validation?
Why you might get asked this:
Validation is a common requirement. This checks your practical approach to ensuring data integrity.
How to answer:
Explain that validation is typically handled manually within the component's state and onChange handlers (for controlled components) or using third-party libraries like Formik or React Hook Form for more complex scenarios.
Example answer:
Forms validation is usually handled by checking input values within the component's state and setting validation errors in the state. For complex forms, libraries like Formik or React Hook Form provide structured ways to manage validation rules and error messages efficiently.
29. What is the role of Redux with React?
Why you might get asked this:
Redux is a popular state management library often used with React. This tests your knowledge of integrating React with external state solutions.
How to answer:
Describe Redux as a predictable state container often used with React to manage application state globally. Explain its core principles (single source of truth, state is read-only, changes made via pure functions - reducers).
Example answer:
Redux is a state management library that helps manage complex application state in a single, predictable store. It's often used with React via the react-redux library to provide global state access to components and manage state changes predictably using actions and reducers.
30. What are lazy loading and code splitting?
Why you might get asked this:
These are performance optimization techniques. Knowing them indicates an understanding of how to improve application load times.
How to answer:
Explain that code splitting divides code into smaller bundles. Lazy loading is loading components or code chunks only when they are needed, usually facilitated by React.lazy and Suspense.
Example answer:
Code splitting is dividing your application's code into smaller bundles that can be loaded on demand. Lazy loading is loading these code bundles/components only when they are required, typically using React.lazy and Suspense in React, which significantly improves initial load performance.
Other Tips to Prepare for a interview questions on react js
Beyond mastering the core concepts covered in common interview questions on React.js, practical preparation is key. Practice coding small React applications, focusing on building components, managing state, and handling events. Work on projects that utilize Hooks, Context API, and potentially a state management library like Redux or Zustand. "The more you practice, the luckier you get," as Arnold Palmer famously said; consistent coding reinforces your understanding. Review recent React features and updates, demonstrating that you stay current in the field. Consider using tools like the Verve AI Interview Copilot to simulate interview scenarios and get feedback on your responses to various interview questions on React js. Its AI-powered analysis can help you refine your explanations and build confidence. Use the Verve AI Interview Copilot at https://vervecopilot.com for targeted practice on specific technologies. Participating in mock interviews, perhaps with peers or mentors, is also invaluable. They can provide feedback on your communication style and identify areas for improvement before you face actual interview questions on React.js. Remember, presenting yourself confidently and articulating your thought process clearly is as important as the technical correctness of your answers. As Thomas Edison noted, "There is no substitute for hard work." Apply dedicated effort to your preparation, including practicing with resources like Verve AI Interview Copilot, and you'll be well-equipped.
Frequently Asked Questions
Q1: How should I structure my answers to React interview questions?
A1: Start with a concise definition, explain its purpose or benefit, and briefly mention implementation details or related concepts.
Q2: Is knowing Hooks mandatory for React interviews now?
A2: Yes, understanding and being able to use React Hooks (like useState, useEffect) is essential for modern React development and expected in interviews.
Q3: Should I prepare for both functional and class components?
A3: Focus primarily on functional components and Hooks, but understand class components and their lifecycle methods as legacy code still exists.
Q4: How can I demonstrate practical experience with React?
A4: Discuss personal projects, contributions to open source, or relevant work experience. Be ready to explain your design choices and challenges faced.
Q5: Are questions about state management libraries like Redux common?
A5: Yes, especially for mid-level and senior roles, as managing state in larger applications is a key concern. Be familiar with at least one solution.
