Top 10 ReactJS Interview Questions

🔥 Top 10 ReactJS Interview Questions You Can’t Afford to Miss! 🚀

Are you preparing for a ReactJS interview? With React’s dominance in front-end development, acing your interview requires more than just basic knowledge. Let’s dive into the top 10 ReactJS interview questions, detailed answers, and examples to help you shine! 💡

1. What is React, and what are its key features?

Answer: React is a JavaScript library (not a framework!) for building dynamic, component-based UIs. Key features:

• Virtual DOM: Efficiently updates the real DOM by comparing virtual snapshots.
• Components: Reusable, self-contained UI elements (class or functional).
• JSX: HTML-like syntax for writing UI logic.
• Unidirectional Data Flow: Props pass data from parent to child.

Example:

// Functional Component with JSX
const Greeting = () => <h1>Hello, React! 👋</h1>;

2. Explain the difference between State and Props.

Answer:

• Props: Immutable data passed from parent to child.
• State: Mutable data managed within a component.

Example:

function Counter() {
  const [count, setCount] = useState(0); // State
  return <button onClick={() => setCount(count + 1)}>Clicks: {count}</button>;
}

// Using Props
<UserProfile name="John" age={25} /> 

3. What are React Lifecycle Methods?

Answer: Class component methods that run at specific phases:

• Mounting: componentDidMount() (after render).
• Updating: componentDidUpdate() (after re-render).
• Unmounting: componentWillUnmount() (before removal).

Example:

class Timer extends React.Component {
  componentDidMount() {
    this.timer = setInterval(() => console.log("Tick ⏰"), 1000);
  }
  componentWillUnmount() {
    clearInterval(this.timer); // Cleanup!
  }
}

4. What are Hooks? Explain useState and useEffect.

Answer: Hooks let functional components use state and lifecycle features.

• useState: Manages local state.
• useEffect: Handles side effects (e.g., API calls).

Example:

function FetchData() {
  const [data, setData] = useState([]);
  useEffect(() => {
    fetchAPI().then(response => setData(response)); 
  }, []); // Runs once on mount
  return <div>{data}</div>;
}

5. How do you handle events in React?

Answer: Use camelCase synthetic events (e.g., onClick, onChange).

Example:

const handleClick = () => alert("Clicked! 🎯");
<button onClick={handleClick}>Click Me</button>

6. Why are Keys important in lists?

Answer: Keys help React identify which items changed, improving performance. Use unique IDs, not indexes!

Example:

const todos = [{ id: 1, text: "Learn React" }, { id: 2, text: "Build Apps" }];
todos.map(todo => <li key={todo.id}>{todo.text}</li>);

7. Controlled vs. Uncontrolled Components

Answer:

• Controlled: Form data handled by React state (recommended).
• Uncontrolled: Data managed by the DOM (using ref).

Example:

// Controlled
const [value, setValue] = useState("");
<input value={value} onChange={(e) => setValue(e.target.value)} />;

// Uncontrolled
const inputRef = useRef();
<input ref={inputRef} />;

8. What is a Higher-Order Component (HOC)?

Answer: A function that takes a component and returns a new component with added functionality.

Example:

function withLogger(WrappedComponent) {
  return function(props) {
    console.log("Rendered:", WrappedComponent.name);
    return <WrappedComponent {...props} />;
  };
}
const EnhancedButton = withLogger(Button);

9. What is React Router?

Answer: A library for handling client-side routing. Key components: BrowserRouter, Route, Link.

Example:

<BrowserRouter>
  <Routes>
    <Route path="/" element={<Home />} />
    <Route path="/about" element={<About />} />
  </Routes>
</BrowserRouter>

10. Explain React Context API.

Answer: A way to share global data without prop drilling. Uses createContext, Provider, and useContext.

Example:

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

🎁 Bonus Tips for Your Interview

1. Master the Latest Features: Know React 18 updates like Concurrent Mode, useId, and Suspense.
2. Practice Coding: Use tools like CodeSandbox or create a small project (e.g., Todo App).
3. Understand React’s Philosophy: Component reusability, unidirectional data flow, and composition.
4. Soft Skills Matter: Communicate your thought process clearly.
5. Ask Questions: Show curiosity about the team’s React practices!

With these questions and tips, you’re ready to ace your React interview! 🚀 Practice hard, stay confident, and keep coding! 💻✨

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