Top 30 Most Common React Js Interview Questions You Should Prepare For
Written by
James Miller, Career Coach
Landing a job as a React developer requires more than just coding skills; you also need to articulate your knowledge effectively during interviews. Hiring managers use interview questions to gauge your understanding of core concepts, your problem-solving abilities, and how you approach building user interfaces with React. Preparing for common questions is crucial for demonstrating your competence and confidence. This guide covers 30 essential react js interview questions, offering insights into what interviewers look for and how to provide clear, concise answers. Mastering these topics will significantly boost your readiness for your next technical interview, whether you are a junior developer or looking to advance your career. Let's dive into the key areas you need to know to succeed.
What Are react js interview questions?
react js interview questions cover a wide range of topics related to the React library. These questions assess your understanding of React's core principles, including components, state, props, and the Virtual DOM. They delve into more advanced concepts like hooks, context, reconciliation, and performance optimization techniques. Interviewers also explore your experience with related libraries like React Router and state management solutions such as Redux. Essentially, react js interview questions are designed to evaluate your theoretical knowledge, practical application skills, and ability to discuss React-specific concepts clearly. Preparing for these questions ensures you can articulate your expertise and demonstrate readiness for a role requiring strong React proficiency.
Why Do Interviewers Ask react js interview questions?
Interviewers ask react js interview questions to determine if a candidate possesses the necessary skills and understanding to build and maintain applications using React. They want to verify theoretical knowledge of React fundamentals and concepts. Questions about the Virtual DOM, JSX, and components assess foundational understanding. Questions on state, props, and data flow evaluate comprehension of how React manages data and renders UI. Advanced topics like hooks, context, and optimization techniques test experience with modern React patterns and performance awareness. Discussing these react js interview questions allows interviewers to gauge problem-solving approaches, debugging skills, and how well a candidate fits into a team environment. Ultimately, it’s about ensuring the candidate can contribute effectively to a React codebase.
What is React?
What are the key features of React?
What is JSX?
What is the Virtual DOM in React?
What are components in React?
What are props in React?
What is state in React?
What is the difference between state and props?
What is the lifecycle of a React component?
What are hooks in React?
What is the difference between controlled and uncontrolled components?
What is reconciliation in React?
How does React handle events?
What is the importance of keys in lists?
What is Redux and how does it relate to React?
What is context in React?
What is the difference between React Router and other routing libraries?
What are Higher-Order Components (HOC)?
What is the significance of shouldComponentUpdate?
What is lazy loading in React?
Explain the concept of reconciliation.
What is prop drilling and how to avoid it?
What is the difference between a pure component and a regular component?
How does React improve SEO?
What is the use of useEffect hook?
How do you optimize React application's performance?
What is the difference between stateful and stateless components?
What is reconciliation in React?
How does React handle form validation?
How to perform automatic redirect after login in React?
Preview List
1. What is React?
Why you might get asked this:
This foundational question assesses your basic understanding of what React is and its purpose as a UI library. It's a standard opener for react js interview questions.
How to answer:
Define React, mentioning its use for building user interfaces, its library status, and its component-based nature.
Example answer:
React is a JavaScript library developed by Facebook for building user interfaces, specifically single-page applications. It allows developers to create reusable UI components and efficiently update the DOM using a virtual representation.
2. What are the key features of React?
Why you might get asked this:
This question tests your awareness of the core elements that define React and differentiate it from other frameworks or libraries.
How to answer:
List and briefly explain the main features like JSX, components, Virtual DOM, one-way data binding, and performance.
Example answer:
Key features include JSX for writing UI, components as reusable building blocks, the Virtual DOM for efficient updates, unidirectional data flow, and its focus on performance by only updating changed parts of the UI.
3. What is JSX?
Why you might get asked this:
JSX is fundamental to writing React code. This question checks if you understand this syntax extension and its role. It's a common topic in react js interview questions.
How to answer:
Define JSX as a syntax extension, explain its purpose (describing UI), and mention that it gets transpiled to JavaScript.
Example answer:
JSX stands for JavaScript XML. It's a syntax extension used in React to write HTML-like code directly within JavaScript, making UI structure declarative and readable. Browsers require it to be transpiled first.
4. What is the Virtual DOM in React?
Why you might get asked this:
Understanding the Virtual DOM is key to grasping how React optimizes performance. This is a crucial conceptual question in react js interview questions.
How to answer:
Explain that it's a lightweight copy of the real DOM, how React uses it for comparison (diffing), and how it improves efficiency by minimizing real DOM manipulations.
Example answer:
The Virtual DOM is an in-memory representation of the real DOM. React uses it to compare the current UI state with the previous one, calculate the minimal changes needed, and efficiently update only those parts in the real DOM.
5. What are components in React?
Why you might get asked this:
Components are the building blocks of React applications. This question verifies your understanding of this core concept.
How to answer:
Define components as reusable UI pieces and mention the two main types: functional (with hooks) and class (with lifecycle methods).
Example answer:
Components are independent and reusable pieces of UI. They can be functional, defined as JavaScript functions and using hooks for state/effects, or class-based, extending React.Component with lifecycle methods.
6. What are props in React?
Why you might get asked this:
Props are essential for passing data down the component tree. This question checks your understanding of data flow.
How to answer:
Explain that props (properties) are read-only inputs passed from parent to child components for configuration and data sharing.
Example answer:
Props are short for properties and are a mechanism for passing data from a parent component down to a child component. They are read-only, enabling child components to be dynamic and reusable.
7. What is state in React?
Why you might get asked this:
Understanding state is crucial for managing dynamic data within a component. This is a frequently asked react js interview questions.
How to answer:
Define state as an object that holds data specific to a component, explain that it's mutable, and mention that changes trigger re-renders.
Example answer:
State is an internal object within a component that holds data that can change over time. Unlike props, state is managed internally by the component and updates to it trigger a re-render of that component.
8. What is the difference between state and props?
Why you might get asked this:
This classic question tests your clear understanding of the fundamental distinction between these two core concepts.
How to answer:
Contrast state and props based on mutability (state is mutable, props are read-only), where they are managed (state in component, props from parent), and their purpose.
Example answer:
Props are external, passed from parent to child, and are immutable. State is internal to a component, managed within it, and is mutable. Props configure a component, while state tracks its dynamic data.
9. What is the lifecycle of a React component?
Why you might get asked this:
Understanding lifecycle methods (or hooks mimicking them) is important for performing actions at specific times in a component's existence.
How to answer:
Mention the main phases (mounting, updating, unmounting) and give examples of methods or hooks used in each phase.
Example answer:
Component lifecycle involves phases like mounting (component creation, e.g., componentDidMount, useEffect with empty dependency array), updating (state/prop changes, e.g., componentDidUpdate, useEffect), and unmounting (removal, e.g., componentWillUnmount, useEffect cleanup).
10. What are hooks in React?
Why you might get asked this:
Hooks are a major feature introduced in React 16.8. This question assesses your knowledge of modern React practices. A key area for react js interview questions.
How to answer:
Explain that hooks are functions enabling state and React features in functional components, listing common examples like useState and useEffect.
Example answer:
Hooks are functions introduced in React 16.8 that allow functional components to use state (useState), perform side effects (useEffect), access context (useContext), and other React features without needing class components.
11. What is the difference between controlled and uncontrolled components?
Why you might get asked this:
This question evaluates your understanding of form handling patterns in React.
How to answer:
Define controlled components as those whose form input values are driven by React state and uncontrolled components as those handling their own state internally (often via refs).
Example answer:
Controlled components have their form input values managed by React state. Uncontrolled components manage their own state internally, often using refs to get their current value directly from the DOM. Controlled components are generally preferred for validation.
12. What is reconciliation in React?
Why you might get asked this:
Reconciliation is React's process for updating the UI efficiently. This question tests your understanding of its internal workings.
How to answer:
Explain that reconciliation is the process of comparing the new Virtual DOM tree with the previous one to identify differences (diffing) and update the real DOM minimally.
Example answer:
Reconciliation is the algorithm React uses to update the UI. It compares the new Virtual DOM tree with the previous one, finds the minimal set of changes required, and applies only those changes to the actual DOM efficiently.
13. How does React handle events?
Why you might get asked this:
Event handling is fundamental to interactive applications. This question checks how you integrate user interactions.
How to answer:
Explain React's synthetic event system, which wraps native browser events for consistency, and mention how event handlers are passed as camelCase props.
Example answer:
React uses a synthetic event system that wraps native browser events, ensuring cross-browser compatibility. Event handlers are attached directly to elements as camelCase props, like onClick or onChange.
14. What is the importance of keys in lists?
Why you might get asked this:
Keys are critical for performance and correct rendering when displaying lists of elements. This is a common pitfall and thus a standard react js interview questions.
How to answer:
Explain that keys help React uniquely identify list items, track changes (additions, removals, reordering), and efficiently update the UI.
Example answer:
Keys are unique identifiers assigned to list items when rendering arrays. They help React efficiently identify which items have changed, been added, or removed, improving rendering performance by minimizing DOM manipulations.
15. What is Redux and how does it relate to React?
Why you might get asked this:
While not part of React, Redux is a very common companion library. This question gauges your experience with state management solutions.
How to answer:
Define Redux as a predictable state container, explain its core principles (single source of truth), and state how it's often used with React for managing complex global state.
Example answer:
Redux is a state management library that provides a single, immutable state tree for the entire application. It relates to React by providing a centralized place to manage complex application-level state that needs to be shared across many components.
16. What is context in React?
Why you might get asked this:
Context is React's built-in solution for passing data without prop drilling. This question tests your knowledge of this feature and its use cases.
How to answer:
Explain that Context provides a way to share values (like theme, user info) across components without explicit prop passing at every level, mentioning createContext and useContext.
Example answer:
React Context provides a way to pass data through the component tree without having to pass props down manually at every level. It's typically used for global data like user authentication, themes, or language settings using createContext and useContext.
17. What is the difference between React Router and other routing libraries?
Why you might get asked this:
React Router is the de facto standard for routing in React. This tests your familiarity with it.
How to answer:
Identify React Router as the standard declarative routing library for React, built specifically for the React ecosystem, handling navigation with components.
Example answer:
React Router is the standard library for handling navigation in React applications. Unlike traditional server-side routing, it enables client-side routing within a single-page application context, using components like and .
18. What are Higher-Order Components (HOC)?
Why you might get asked this:
HOCs are a pattern for code reuse and logic abstraction in class components. This question checks your knowledge of this pattern.
How to answer:
Define an HOC as a function that takes a component and returns a new component with enhanced functionality, stating its use for code reuse and logic sharing.
Example answer:
A Higher-Order Component (HOC) is an advanced technique in React for reusing component logic. It's a function that takes a component as an argument and returns a new component with additional props or behavior.
19. What is the significance of shouldComponentUpdate?
Why you might get asked this:
This lifecycle method is a key optimization technique for class components. It demonstrates your awareness of performance tuning.
How to answer:
Explain that shouldComponentUpdate allows you to manually control whether a component re-renders by returning true or false, preventing unnecessary updates.
Example answer:
shouldComponentUpdate is a lifecycle method in class components used for performance optimization. By implementing it, you can prevent the component from re-rendering if its props or state haven't changed significantly, based on specific logic.
20. What is lazy loading in React?
Why you might get asked this:
Lazy loading (code splitting) is a common optimization technique. This question assesses your knowledge of improving initial load times.
How to answer:
Define lazy loading as delaying component loading until needed, mention its benefit (faster initial load), and the tools used (React.lazy and Suspense).
Example answer:
Lazy loading, or code splitting, is a technique to load component code only when it's needed, rather than all at once. This improves initial loading performance. In React, this is done using React.lazy and Suspense.
21. Explain the concept of reconciliation.
Why you might get asked this:
This is a rephrasing of question 12, often used to see if you can explain the concept in different terms or elaborate further.
How to answer:
Reinforce the explanation of the Virtual DOM diffing process and the subsequent minimal updates to the real DOM.
Example answer:
Reconciliation is React's process of determining how to efficiently update the UI. It builds a new Virtual DOM tree after a state or prop change, compares it to the old one using a diffing algorithm, and applies only the necessary changes to the real DOM.
22. What is prop drilling and how to avoid it?
Why you might get asked this:
Prop drilling is a common problem in component hierarchies. This question tests your awareness of it and solutions.
How to answer:
Define prop drilling as passing props through many intermediate components that don't directly use them, and suggest avoiding it using Context or state management libraries.
Example answer:
Prop drilling is when you pass data down through multiple nested components via props, even if intermediate components don't need that data. It can be avoided using React Context for localized sharing or state management libraries like Redux for global state.
23. What is the difference between a pure component and a regular component?
Why you might get asked this:
Pure Components (and React.memo for functional) are optimization tools. This tests your knowledge of them.
How to answer:
Explain that React.PureComponent (for class) or React.memo (for functional) automatically implement shallow prop and state comparison to prevent unnecessary re-renders, unlike regular components.
Example answer:
A pure component (via React.PureComponent or React.memo) automatically performs a shallow comparison of its props and state. If the shallow comparison shows no changes, it skips the re-render, unlike a regular component which always re-renders by default.
24. How does React improve SEO?
Why you might get asked this:
Client-side rendering can hurt SEO. This question checks if you know how React addresses this.
How to answer:
Explain that React supports Server-Side Rendering (SSR), which allows the initial page HTML to be rendered on the server, making it crawlable by search engines.
Example answer:
React improves SEO primarily through Server-Side Rendering (SSR). By rendering the initial HTML on the server, search engine crawlers can easily access and index the page content, which is often difficult with purely client-side rendered SPAs.
25. What is the use of useEffect hook?
Why you might get asked this:
useEffect is fundamental to functional components. This is a critical hook-related react js interview questions.
How to answer:
Explain that useEffect allows performing side effects (data fetching, subscriptions, DOM manipulation) in functional components and can replicate lifecycle behaviors.
Example answer:
The useEffect hook is used in functional components to perform side effects, such as fetching data, subscribing to events, or directly interacting with the DOM. It combines the functionality of lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount.
26. How do you optimize React application's performance?
Why you might get asked this:
Performance is a key concern in large applications. This tests your practical knowledge of making apps faster.
How to answer:
List several optimization techniques like using React.memo or PureComponent, lazy loading, using keys, avoiding unnecessary re-renders, and using the React Profiler.
Example answer:
Performance optimization in React includes using React.memo (or PureComponent) to prevent unnecessary re-renders, implementing code splitting/lazy loading (React.lazy, Suspense), using correct keys for lists, optimizing state updates, and utilizing performance profiling tools.
27. What is the difference between stateful and stateless components?
Why you might get asked this:
This classic distinction highlights whether a component manages its own data or just presents data given to it.
How to answer:
Define stateful components as those that manage internal state (using useState or this.state) and stateless components as those that receive data via props and don't manage state themselves.
Example answer:
Stateful components manage their own data using useState (functional) or this.state (class). Stateless components, often functional, receive all data via props and simply render UI based on that data, without holding internal state.
28. What is reconciliation in React?
Why you might get asked this:
Another opportunity to explain reconciliation, potentially prompting for more detail or examples. Reinforce your understanding.
How to answer:
Reiterate the Virtual DOM diffing process and efficient DOM updates, perhaps adding a note about the Fiber algorithm (React's current reconciliation engine).
Example answer:
Reconciliation is React's core process for figuring out how to update the UI efficiently when state or props change. It involves comparing the new Virtual DOM tree with the old one and applying the minimum number of operations needed to update the actual DOM.
29. How does React handle form validation?
Why you might get asked this:
Form handling and validation are common tasks. This checks your approach within the React paradigm.
How to answer:
Explain that validation is typically done using controlled components, where input values are in state, and validation logic is triggered on input change or form submission.
Example answer:
React handles form validation typically by using controlled components. Input values are stored in the component's state, and validation logic is run either onChange as the user types or onSubmit when the form is submitted, updating state to show error messages.
30. How to perform automatic redirect after login in React?
Why you might get asked this:
This is a practical, common use case. It tests your knowledge of navigation libraries like React Router.
How to answer:
Explain how to use react-router-dom's navigate function (from useNavigate hook) or the component.
Example answer:
Using react-router-dom, after a successful login, you can use the navigate function obtained from the useNavigate hook: const navigate = useNavigate(); navigate('/dashboard');. Alternatively, you can render a component.
Other Tips to Prepare for a react js interview questions
Beyond memorizing answers to specific react js interview questions, effective preparation involves practical steps. Practice coding by building small projects or contributing to open source; this solidifies your understanding and provides tangible examples for discussion. Review fundamental JavaScript concepts, as React heavily relies on them. "Coding is a craft; mastery comes through practice," as the saying goes. Be prepared to discuss your past projects, explaining your role, the challenges you faced, and how you used React to solve them. Use the STAR method (Situation, Task, Action, Result) to structure your project descriptions. Consider using tools like the Verve AI Interview Copilot to practice answering react js interview questions and receive instant feedback. The Verve AI Interview Copilot at https://vervecopilot.com can help you refine your responses and build confidence. Engaging in mock interviews, perhaps facilitated by platforms offering AI-powered practice like the Verve AI Interview Copilot, is invaluable for simulating the actual interview environment. Remember, confidence and clear communication are just as important as technical knowledge when tackling react js interview questions.
Frequently Asked Questions
Q1: What is a component prop?
A1: A component prop is when you pass an entire React component or element as a prop to another component for rendering.
Q2: How do you use fragments?
A2: Fragments ( or <>...) allow grouping multiple elements without adding extra nodes to the DOM.
Q3: What is server-side rendering (SSR)?
A3: SSR renders React components to HTML on the server, sending the fully rendered page to the client, improving initial load and SEO.
Q4: What is the purpose of strict mode?
A4: React.StrictMode is a development tool that checks for potential problems in an application, like legacy API usage or unsafe lifecycles.
Q5: What are controlled components?
A5: Controlled components are form inputs whose values are entirely managed by React state, making state the 'single source of truth'.
Q6: How do you test React components?
A6: React components are typically tested using libraries like Jest for running tests and React Testing Library or Enzyme for rendering and interacting with components.
