React is one of the most popular JavaScript libraries for building modern web applications. Particularly for frontend developer positions, React knowledge is often a key focus during interviews. In this article, we’ve compiled 10 essential questions with sample answers that you might encounter at both junior and senior levels. Here’s some knowledge that will give you an edge in your React interviews!

1. What is React and What are Its Advantages?

This question is a fundamental starting point, especially for junior candidates. To demonstrate your knowledge of React, you can mention the following points:

• React is an open-source JavaScript library developed by Facebook.
• Its component-based architecture reduces code duplication and accelerates application development.
• React optimizes performance using the Virtual DOM. The Virtual DOM applies changes first on a virtual copy and then only updates the real DOM with the necessary changes, significantly improving performance.
• React offers extensive community support and rich resources. As it is used by millions of developers globally, you'll find plenty of resources to help resolve any issues.

2. What is JSX and How Does it Work?

JSX is one of the fundamental features of React. This question is important to demonstrate your understanding of how React works:

• JSX stands for JavaScript XML and allows you to create React components using a syntax that looks similar to HTML.
• JSX is not directly understood by browsers, so it must be converted to JavaScript using a transpiler like Babel.
   

Example JSX code:

const element = <h1>Hello, Techcareer.net!</h1>;

JSX allows you to leverage all the power of JavaScript. For instance, you can use JavaScript expressions within curly braces:

const name = "Alperen";
const element = <h1>Hello, {name}!</h1>;

3. What is the Difference Between State and Props?

In React, state and props are two fundamental concepts for transferring data between components:

• State: Manages a component’s internal state. It is mutable and can be changed within the component. For example, state is used to track a user's input in a form.
• Props: Are external data passed into a component. Props are immutable (read-only) and cannot be changed. For example, props are used to pass data from one component to another.

Example:

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

4. What are React Lifecycle Methods?

The lifecycle of React components is frequently asked, especially in senior-level interviews. The main lifecycle methods are:

• componentDidMount: Runs when the component is added to the DOM. For instance, API calls can be made in this method.
• componentDidUpdate: Runs when the component is updated. It is triggered when state or props change.
• componentWillUnmount: Runs just before the component is removed from the DOM. It is commonly used to clean up subscriptions.
• useEffect Hook: Manages lifecycle behavior in functional components.

Example:

useEffect(() => {
 console.log("Component mounted!");
 return () => {
   console.log("Component unmounted!");
 };
}, []);

5. What are React Hooks and How Do You Use Them?

React Hooks allow you to use state and lifecycle features in functional components. The most commonly used hooks are:

• useState: Used for state management.

Example:

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

• useEffect: Used for managing side effects such as API calls or DOM manipulations.
• useContext: Provides global state management with the Context API.
   

Example:

const theme = useContext(ThemeContext);

6. What is Redux and When Should You Use It?

Redux is a library that simplifies state management in React applications. This question is typically asked to senior-level candidates:

• Redux manages application state in a central store.
• You should be familiar with core concepts like actions, reducers, and the store.
• Redux is useful for managing state in large-scale applications. For example, it can be used to manage items in a shopping cart in an e-commerce app.

7. Why is the Key Prop Important in React?

When rendering lists in React, the key prop is used to help React identify which items have changed, been added, or removed. Keys must be unique for each element.

Example:

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

8. How Do You Optimize Performance in React?

Performance optimization in React is a critical topic for senior candidates:

• React.memo: Prevents unnecessary re-renders of components.
• useMemo and useCallback: Cache computations and functions to optimize performance.
• Lazy Loading: Loads components only when needed.

Example:

const LazyComponent = React.lazy(() => import('./LazyComponent'));

9. What is the Context API and How Do You Use It?

The Context API is a feature in React for global state management.

• useContext Hook: Simplifies state management with the Context API.

Example:

const ThemeContext = React.createContext('light');
function App() {
 return (
   <ThemeContext.Provider value="dark">
     <Toolbar />
   </ThemeContext.Provider>
 );
}

10. What is React Router and How Do You Use It?

React Router is a library for managing page navigation in React applications. It is important to learn core components such as BrowserRouter, Route, Switch, and Link.

Example:

import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';

function App() {
 return (
   <Router>
     <Switch>
       <Route path="/about" component={About} />
       <Route path="/" component={Home} />
     </Switch>
   </Router>
 );
}

React interview questions are designed to test both your foundational knowledge and in-depth understanding of the library. In this guide, we've shared 10 essential React interview questions and sample answers that you may encounter at both junior and senior levels. If you're preparing for these questions, be sure to check out more resources on Techcareer.net to further enhance your skills.

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