perf: replace delete with = undefined in request hot paths#91560
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benfavre wants to merge 2 commits intovercel:canaryfrom
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perf: replace delete with = undefined in request hot paths#91560benfavre wants to merge 2 commits intovercel:canaryfrom
benfavre wants to merge 2 commits intovercel:canaryfrom
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…classes Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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delete on hot-path objects to preserve V8 hidden classes…ects The query object is exposed to user code (getServerSideProps, middleware, etc.). Setting internal keys to undefined instead of deleting them would leak those keys via Object.keys(), for...in, and Object.entries() — even though the values are undefined. Only keep the change for internal-only objects (request headers, request metadata) where enumeration isn't observable.
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The delete→undefined changes for NEXT_URL header and request-meta are already in PR vercel#91560. Keep only the changes unique to this PR: - parsedUrl.query fresh object replacement (avoids N delete operations) - loadable timer guards during SSR
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Summary
Replace
delete obj.propwithobj.prop = undefinedin server request-handling hot paths where the object is not exposed to user code, to preserve V8 hidden classes.Problem
The
deleteoperator changes an object's hidden class (shape), forcing V8 into slower dictionary mode. TanStack Start saw >50% CPU time reduction in affected methods.Changes
= undefined(safe: IncomingHttpHeaders not exposed to user iteration)NEXT_URLheader cleared with= undefined(safe: internal header)removeRequestMetauses= undefined(safe: internal metadata accessed only viagetRequestMeta())NOT changed (intentionally)
Query object keys (
_rsc,nextInternalLocale,__next_*prefixed params) must usedeletebecause the query object is exposed to user code. Setting toundefinedwould leak internal keys viaObject.keys(),for...in, andObject.entries().Test plan
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