What is the use of useEffect React Hooks?

useEffect is a built-in React Hook that lets you perform side effects in functional components—such as data fetching, subscriptions, DOM mutations, and cleanup—by running a function after render.

1. Runs After Render

   • By default, the effect callback executes after every completed render, keeping UI in sync with state or props.
   • You can control when it runs by providing a dependency array.

2. Dependency Array 📋

   • useEffect(fn, []) runs once after the initial render (like componentDidMount).
   • useEffect(fn, [dep1, dep2]) runs on initial render and whenever any dependency changes (like componentDidUpdate).
   • Omitting the array runs the effect after every render.

3. Cleanup Function 🧹

   • If your effect creates subscriptions, timers, or external listeners, return a cleanup function:

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4. Common Use Cases

   • Data fetching (API calls)
   • Subscribing/unsubscribing to events or WebSockets
   • Manually updating the DOM (e.g., setting document.title)
   • Timers and intervals