vercel-react-best-practices

---

title: Narrow Effect Dependencies impact: LOW impactDescription: minimizes effect re-runs tags: rerender, useEffect, dependencies, optimization

Narrow Effect Dependencies

Specify primitive dependencies instead of objects to minimize effect re-runs.

Incorrect (re-runs on any user field change):

useEffect(() => {
  fetchUserDetails(user.id)
}, [user])

Correct (re-runs only when id changes):

useEffect(() => {
  fetchUserDetails(user.id)
}, [user.id])

For derived state, compute outside effect:

// Incorrect: runs on width=767, 766, 765...
useEffect(() => {
  if (width < 768) {
    enableMobileMode()
  }
}, [width])

// Correct: runs only on boolean transition
const isMobile = width < 768
useEffect(() => {
  if (isMobile) {
    enableMobileMode()
  }
}, [isMobile])