React Developer Interview Questions with Answers

React has become the go-to JavaScript library for building fast, interactive, and scalable user interfaces. Whether you’re a fresher aiming for your first front-end role or an experienced developer transitioning into a React-based tech stack, preparing with the right set of React interview questions can give you a solid edge in technical interviews.

In this blog, we’ll walk you through a carefully selected list of React interview questions along with simple, understandable answers. These questions cover both the fundamentals and some advanced concepts to help you confidently face your next interview.

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1. What is React and why is it used?

React is a JavaScript library developed by Facebook for building user interfaces, especially for single-page applications. It allows developers to create reusable UI components, manage state efficiently, and render components dynamically.

Why use React?

• Component-based architecture

• Virtual DOM for performance

• Strong community support

• Easy to integrate with other libraries or frameworks

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2. What is JSX in React?

JSX (JavaScript XML) is a syntax extension for JavaScript that looks similar to HTML. It is used with React to describe what the UI should look like.

Example:

jsx
const element = <h1>Hello, React!</h1>;

JSX makes the code more readable and easier to write, but it gets compiled into standard JavaScript under the hood using tools like Babel.

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3. What is the Virtual DOM and how does React use it?

The Virtual DOM is a lightweight copy of the real DOM. React uses it to optimize rendering performance by keeping a virtual representation of the UI in memory. When a change occurs, React compares the new virtual DOM with the previous one (a process called diffing), and efficiently updates only the changed parts of the real DOM.

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4. What are components in React?

React components are the building blocks of any React application. They allow you to split the UI into independent, reusable pieces.

Types of components:

• Functional Components: Simple functions that return JSX.

• Class Components: ES6 classes that extend React.Component and include lifecycle methods.

Since the introduction of hooks, functional components are preferred due to their simplicity and readability.

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5. What are props in React?

Props (short for properties) are inputs passed from a parent component to a child component. They are read-only and used to customize or configure child components.

Example:

jsx
function Greeting(props) {
  return <h1>Hello, {props.name}!</h1>;
}

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6. What is state in React?

State is a built-in object in React used to store dynamic data that determines the component’s behavior and rendering. Unlike props, state is managed within the component and can be changed using the useState hook (in functional components) or this.setState (in class components).

Example with useState:

jsx
const [count, setCount] = useState(0);

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7. What are React Hooks?

Hooks are functions introduced in React 16.8 that allow you to use state and lifecycle features in functional components.

Common hooks:

• useState – for managing state

• useEffect – for side effects (like API calls)

• useContext – to use React Context

• useRef – to access DOM elements or persist values

Hooks make functional components more powerful and easier to maintain.

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8. What is the difference between controlled and uncontrolled components?

• Controlled Components: Form data is handled by React state.

• Uncontrolled Components: Form data is handled by the DOM directly via refs.

Controlled components offer more control over form validation and logic.

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9. What is React Context API?

The Context API allows you to share state globally across the entire app without having to pass props manually at every level.

Useful for:

• Themes

• User authentication

• Language localization

Example:

jsx
const UserContext = React.createContext();

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10. What is Redux and how does it relate to React?

Redux is a state management library often used with React to handle complex state across multiple components. It uses a single source of truth (a global store), and actions are dispatched to modify the state through reducers.

Although Redux is powerful, newer React features like Context API and useReducer have made it possible to manage state in simpler ways for smaller applications.

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11. What is useEffect and when is it used?

useEffect is a React hook used to perform side effects in functional components, such as data fetching, timers, and manual DOM manipulation.

Example:

jsx
useEffect(() => {
  console.log('Component mounted');
}, []);

The empty dependency array [] ensures the effect runs only once when the component mounts.

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12. How does React handle forms?

React handles forms by storing form data in component state and updating it on every change.

Example:

jsx
<input type="text" value={name} onChange={(e) => setName(e.target.value)} />

This creates a controlled component where the input value is always in sync with the state.

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13. What are keys in React and why are they important?

Keys are special attributes used in lists to identify each element uniquely. They help React determine which items have changed, are added, or removed.

Always use a unique and stable value, like an ID.

Example:

jsx
{items.map(item => <li key={item.id}>{item.name}</li>)}

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 Final Thoughts

Whether you’re preparing for a startup interview or applying to a big tech company, having a strong understanding of these React interview questions can help you stand out. Focus on mastering core concepts like components, state, props, hooks, and lifecycle management.

The key is not just memorizing answers, but truly understanding how React works under the hood and being able to explain your reasoning during technical discussions.

By practicing these React interview questions, building small projects, and staying updated with the latest features in React, you'll be well on your way to landing that dream role as a React Developer.