Server Dedup Props

Source: .agents/references/coding-standard/vercel-react-best-practices/rules/server-dedup-props.md

Metadata

• title: Avoid Duplicate Fields in Page Props
• impact: LOW
• impactDescription: reduces network payload by avoiding duplicate serialization
• tags: server, pages-router, serialization, props, getStaticProps, getServerSideProps

Content

This repo: Pages Router only — avoid returning overlapping or derived copies of the same data from getStaticProps / getServerSideProps. Do expensive transforms client-side when the page can derive them from one source.

Avoid Duplicate Fields in Page Props

Impact: LOW (reduces payload size in __NEXT_DATA__)

Each distinct value in props is serialized. Returning the same logical data twice (raw array + sorted copy, full object + single field) doubles payload size.

Incorrect (duplicates array in props):

export const getServerSideProps: GetServerSideProps = async () => {
  const usernames = await fetchUsernames()
  return {
    props: {
      usernames,
      usernamesOrdered: usernames.toSorted()
    }
  }
}

Correct (send once; sort on client):

export const getServerSideProps: GetServerSideProps = async () => {
  const usernames = await fetchUsernames()
  return { props: { usernames } }
}

// Page: derive sorted list during render or useMemo
function Page({ usernames }: { usernames: string[] }) {
  const sorted = useMemo(() => [...usernames].sort(), [usernames])
  // ...
}

Operations that commonly duplicate payload when returned twice:

• Arrays: .toSorted(), .filter(), .map(), .slice(), [...arr]
• Objects: {...obj}, Object.assign(), structuredClone()

More examples:

// Avoid
return { props: { users, activeUsers: users.filter((u) => u.active) } }
return { props: { product, productName: product.name } } }

// Prefer
return { props: { users } }
return { props: { product } }
// Filter or destructure in the page component

Exception: Return derived data from the data function when the transform is expensive and the client cannot reuse the original efficiently.