aiworker/.codex/skills/vercel-react-best-practices/rules/server-cache-react.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Per-Request Deduplication with React.cache()	MEDIUM	deduplicates within request	server, cache, react-cache, deduplication

Per-Request Deduplication with React.cache()

Use React.cache() for server-side request deduplication. Authentication and database queries benefit most.

Usage:

import { cache } from 'react'

export const getCurrentUser = cache(async () => {
  const session = await auth()
  if (!session?.user?.id) return null
  return await db.user.findUnique({
    where: { id: session.user.id }
  })
})

Within a single request, multiple calls to getCurrentUser() execute the query only once.

Avoid inline objects as arguments:

React.cache() uses shallow equality (Object.is) to determine cache hits. Inline objects create new references each call, preventing cache hits.

Incorrect (always cache miss):

const getUser = cache(async (params: { uid: number }) => {
  return await db.user.findUnique({ where: { id: params.uid } })
})

// Each call creates new object, never hits cache
getUser({ uid: 1 })
getUser({ uid: 1 })  // Cache miss, runs query again

Correct (cache hit):

const getUser = cache(async (uid: number) => {
  return await db.user.findUnique({ where: { id: uid } })
})

// Primitive args use value equality
getUser(1)
getUser(1)  // Cache hit, returns cached result

If you must pass objects, pass the same reference:

const params = { uid: 1 }
getUser(params)  // Query runs
getUser(params)  // Cache hit (same reference)

Next.js-Specific Note:

In Next.js, the fetch API is automatically extended with request memoization. Requests with the same URL and options are automatically deduplicated within a single request, so you don't need React.cache() for fetch calls. However, React.cache() is still essential for other async tasks:

• Database queries (Prisma, Drizzle, etc.)
• Heavy computations
• Authentication checks
• File system operations
• Any non-fetch async work

Use React.cache() to deduplicate these operations across your component tree.

Reference: React.cache documentation