qcast-front/.agents/skills/vercel-react-best-practices/rules/rerender-split-combined-hooks.md

title	impact	impactDescription	tags
Split Combined Hook Computations	MEDIUM	avoids recomputing independent steps	rerender, useMemo, useEffect, dependencies, optimization

Split Combined Hook Computations

When a hook contains multiple independent tasks with different dependencies, split them into separate hooks. A combined hook reruns all tasks when any dependency changes, even if some tasks don't use the changed value.

Incorrect (changing sortOrder recomputes filtering):

const sortedProducts = useMemo(() => {
  const filtered = products.filter((p) => p.category === category)
  const sorted = filtered.toSorted((a, b) =>
    sortOrder === "asc" ? a.price - b.price : b.price - a.price
  )
  return sorted
}, [products, category, sortOrder])

Correct (filtering only recomputes when products or category change):

const filteredProducts = useMemo(
  () => products.filter((p) => p.category === category),
  [products, category]
)

const sortedProducts = useMemo(
  () =>
    filteredProducts.toSorted((a, b) =>
      sortOrder === "asc" ? a.price - b.price : b.price - a.price
    ),
  [filteredProducts, sortOrder]
)

This pattern also applies to useEffect when combining unrelated side effects:

Incorrect (both effects run when either dependency changes):

useEffect(() => {
  analytics.trackPageView(pathname)
  document.title = `${pageTitle} | My App`
}, [pathname, pageTitle])

Correct (effects run independently):

useEffect(() => {
  analytics.trackPageView(pathname)
}, [pathname])

useEffect(() => {
  document.title = `${pageTitle} | My App`
}, [pageTitle])

Note: If your project has React Compiler enabled, it automatically optimizes dependency tracking and may handle some of these cases for you.