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import {useEffect} from "react";

The statement import {useEffect} from "react"; is a JavaScript import declaration used in React applications.

Purpose:
This line imports the useEffect Hook from the React library. The useEffect Hook is a fundamental part of functional components in React, enabling developers to perform "side effects."

Side Effects in React:
Side effects are operations that interact with the outside world or have an impact beyond the component's render cycle. Common examples of side effects include:
  • Data fetching: Making API calls to retrieve data from a server.
  • DOM manipulation: Directly interacting with the browser's Document Object Model (e.g., setting the document title).
  • Subscriptions: Setting up event listeners or connecting to external services like WebSockets.
  • Timers: Using setTimeout or setInterval.
How useEffect Works:

useEffect takes two arguments:
  • A setup function:
    This function contains the code for your side effect. It runs after every render of the component (including the initial render) unless its dependencies prevent it from doing so.
  • A dependency array (optional):
    This array specifies the values that the effect depends on. If any of these values change between renders, the effect will re-run. An empty dependency array [] ensures the effect runs only once after the initial render (similar to componentDidMount in class components).
Example Usage: 

import { useEffect, useState } from "react";

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

  useEffect(() => {
    // This side effect updates the document title
    document.title = `Count: ${count}`;
  }, [count]); // The effect re-runs whenever 'count' changes

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

export default MyComponent;
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