useEffect in React
From the React Hooks cheat sheet · Side Effects · verified Jul 2026
useEffect
Side effects and lifecycle
javascript
// After every render
useEffect(() => {
console.log('Rendered')
})
// Once on mount
useEffect(() => {
fetchData()
}, [])
// When dependencies change
useEffect(() => {
const timer = setTimeout(() => {}, 1000)
return () => clearTimeout(timer) // Cleanup
}, [count, id])⚠️ Always include dependencies or ESLint will warn
💡 Return cleanup function for subscriptions
✅ Empty deps [] = componentDidMount