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Understanding these hooks is crucial for managing state and side effects in your React applications. They provide a powerful way to build dynamic and responsive UIs with functional components.

useState

The useState hook allows you to add state to functional components. Here's a simple example demonstrating how to use useState:

Counter Example

import React, { useState } from 'react';

function Counter() {
  // Declare a state variable named "count" and a function "setCount" to update it
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

export default Counter;

In this example:

• useState(0) initializes the count state variable with 0.
• setCount is a function that updates the state.
• Clicking the button increments the count state by 1, which causes the component to re-render and display the new count.

useEffect

The useEffect hook allows you to perform side effects in function components. Side effects can include data fetching, subscriptions, or manually changing the DOM.

Example: Data Fetching

Here's an example that uses useEffect to fetch data from an API when the component mounts:

import React, { useState, useEffect } from 'react';

function DataFetcher() {
  const [data, setData] = useState([]);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    // Fetch data from an API
    fetch('https://jsonplaceholder.typicode.com/posts')
      .then(response => response.json())
      .then(data => {
        setData(data);
        setLoading(false);
      });
  }, []); // Empty dependency array means this effect runs once after initial render

  if (loading) {
    return <div>Loading...</div>;
  }

  return (
    <div>
      <h1>Fetched Data</h1>
      <ul>
        {data.map(item => (
          <li key={item.id}>{item.title}</li>
        ))}
      </ul>
    </div>
  );
}

export default DataFetcher;

In this example:

• useEffect is used to fetch data when the component mounts.
• The empty dependency array ([]) ensures that the effect runs only once after the initial render.
• The fetched data is stored in the data state variable, and the loading state variable is used to show a loading message until the data is fetched.

useEffect Dependencies

The useEffect hook can also accept a dependency array that allows you to control when the effect runs.

Example: Effect with Dependencies

import React, { useState, useEffect } from 'react';

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

  useEffect(() => {
    // Update the document title using the browser API
    document.title = `You clicked ${count} times`;

    // Clean up by resetting the title when component unmounts
    return () => {
      document.title = 'React App';
    };
  }, [count]); // Only re-run the effect if count changes

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>
        Click me
      </button>
    </div>
  );
}

export default CounterWithEffect;

In this example:

• The effect updates the document title whenever the count state changes.
• The dependency array [count] ensures that the effect only runs when count changes.
• The cleanup function resets the document title when the component unmounts or before the effect runs again.

Conclusion

• useState is used for adding state to function components. It returns an array containing the current state value and a function to update it.
• useEffect is used for handling side effects in function components. It runs after the component renders and can optionally clean up before the component unmounts or before the effect runs again.

Happy Coding !