Apa iku useEffect lan kapan dheweke mlaku?

Question

Accepted Answer

`useEffect` mlaku **side effects** — gawe sing metu saka React's rendering, kayata data fetching, subscriptions, timers, utawa manual DOM updates. Dheweke mlaku **sawisé** component ngrender, ngekno side effects metu saka render body (render kudu tetep pure).

## Dependency array ngontrol timing

```jsx
useEffect(() => {
  document.title = `Count: ${count}`;
}, [count]); // ← dependency array
```

- **`[count]`** → mlaku sawisé first render lan saben `count` owah.
- **`[]`** (kosong) → mlaku **sapakale** sawisé mount (apik kanggo setup sapakale).
- **ditinggal** → mlaku sawisé **saben** render (jarang sing dikarepake).

## Cleanup: mbalika function

Yen effect nyetel soko sing kudu diruwat, mbalika cleanup function. React nelpon dheweke sadurunge next effect mlaku lan nalika component unmounts:

```jsx
useEffect(() => {
  const id = setInterval(() => setN(n => n + 1), 1000);
  return () => clearInterval(id); // cleanup → prevents a leaked timer
}, []);
```

## Napa cleanup penting

Tanpa dheweke, saben re-run bakal nambah timer/subscription liyane lan ora tau mbusak sing lawas — memory leak klasik. Lifecycle kanggo `[dep]` effect iku: *mlaku effect → (dep owah) → mlaku cleanup → mlaku effect maneh*.

**Tip:** ora nggunakake effects kangga barang sing bisa kowe compute sajrone render (derive values dadi), lan isoa saben nilai sing effect maca ing dependency array (`react-hooks/exhaustive-deps` lint rule tulung).