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perf: eliminate structuredClone in metadata resolution (914ms → ~0) #91577

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perf: eliminate structuredClone in metadata resolution (914ms → ~0) #91577

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f328943
perf: replace structuredClone with shallow copy in metadata resolution
benfavre Mar 18, 2026
dc37068
fix: deep-copy mutable nested objects in metadata resolution
benfavre Mar 18, 2026
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25 changes: 23 additions & 2 deletions packages/next/src/lib/metadata/resolve-metadata.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -232,7 +232,26 @@ async function mergeMetadata(
leafSegmentStaticIcons: StaticIcons
}
): Promise<ResolvedMetadata> {
const newResolvedMetadata = structuredClone(resolvedMetadata)
// Shallow-copy the top-level object, plus a targeted copy of nested objects
// that postProcessMetadata / inheritFromMetadata may mutate in-place
// (e.g. icons.icon.unshift, openGraph.title = ..., twitter Object.assign).
// A plain `{ ...resolvedMetadata }` would keep frozen inner references in
// dev mode (deepFreeze) and cause shared-state leaks in production.
const newResolvedMetadata: ResolvedMetadata = {
...resolvedMetadata,
openGraph: resolvedMetadata.openGraph
? { ...resolvedMetadata.openGraph }
: null,
twitter: resolvedMetadata.twitter
? { ...resolvedMetadata.twitter }
: null,
icons: resolvedMetadata.icons
? {
icon: [...resolvedMetadata.icons.icon],
apple: [...resolvedMetadata.icons.apple],
}
: null,
}

const metadataBase = normalizeMetadataBase(
metadata?.metadataBase !== undefined
Expand Down Expand Up @@ -456,7 +475,9 @@ function mergeViewport({
resolvedViewport: ResolvedViewport
viewport: Viewport | null
}): ResolvedViewport {
const newResolvedViewport = structuredClone(resolvedViewport)
// Use a shallow copy instead of structuredClone. This is safe because
// every property below is fully replaced (never mutated in-place).
const newResolvedViewport: ResolvedViewport = { ...resolvedViewport }

if (viewport) {
for (const key_ in viewport) {
Expand Down
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