What is the use of useEffect()?

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The useEffect() hook in React is used to handle side effects in functional components. A side effect is anything that affects something outside the component’s scope, such as fetching data from an API, updating the document title, subscribing/unsubscribing to events, or manipulating the DOM.

In class components, side effects were managed with lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount. With hooks, useEffect() combines all of these into one API.

Syntax:

useEffect(() => {
  // code to run (effect)
  return () => {
    // optional cleanup
  };
}, [dependencies]);

Key Uses:

1. Run on every render (no dependency array):

   useEffect(() => {
     console.log("Runs after every render");
   });
   

2. Run only once (on mount) (empty dependency array []):

   useEffect(() => {
     console.log("Runs only once when component mounts");
   }, []);
   

3. Run when specific state/props change:

   useEffect(() => {
     console.log("Runs when count changes");
   }, [count]);
   

4. Cleanup side effects (unsubscribe/remove listeners):

   useEffect(() => {
     const timer = setInterval(() => console.log("Tick"), 1000);
     return () => clearInterval(timer); // cleanup
   }, []);
   

✅ In short: useEffect() keeps functional components reactive by syncing them with external systems (APIs, events, browser APIs), ensuring proper setup and cleanup.

Read More:

What are promises?

What are props and state?

What are React Hooks?

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