perf: replace delete with undefined assignment to preserve V8 hidden classes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: benfavre

packages/next/src/server/app-render/strip-flight-headers.ts

@@ -9,6 +9,6 @@ import { FLIGHT_HEADERS } from '../../client/components/app-router-headers'
  */
 export function stripFlightHeaders(headers: IncomingHttpHeaders) {
   for (const header of FLIGHT_HEADERS) {
-    delete headers[header]
+    headers[header] = undefined
   }
 }

packages/next/src/server/base-server.ts

@@ -2018,7 +2018,7 @@ export default abstract class Server<
     if (!addedNextUrlToVary) {
       // Remove `Next-URL` from the request headers we determined it wasn't necessary to include in the Vary header.
       // This is to avoid any dependency on the `Next-URL` header being present when preparing the response.
-      delete req.headers[NEXT_URL]
+      req.headers[NEXT_URL] = undefined
     }
   }
 
@@ -2579,7 +2579,7 @@ export default abstract class Server<
         query[NEXT_RSC_UNION_QUERY]
       )
     }
-    delete query[NEXT_RSC_UNION_QUERY]
+    query[NEXT_RSC_UNION_QUERY] = undefined
 
     const options: MatchOptions = {
       i18n: this.i18nProvider?.fromRequest(req, pathname),

packages/next/src/server/next-server.ts

@@ -1119,7 +1119,7 @@ export default class NextNodeServer extends BaseServer<
           await this.render404(req, res, parsedUrl)
           return true
         }
-        delete query[NEXT_RSC_UNION_QUERY]
+        query[NEXT_RSC_UNION_QUERY] = undefined
 
         // If we handled the request, we can return early.
         // For api routes edge runtime

packages/next/src/server/request-meta.ts

@@ -372,7 +372,7 @@ export function removeRequestMeta<K extends keyof RequestMeta>(
   key: K
 ) {
   const meta = getRequestMeta(request)
-  delete meta[key]
+  meta[key] = undefined as RequestMeta[K]
   return setRequestMeta(request, meta)
 }
 

packages/next/src/server/server-utils.ts

@@ -34,7 +34,7 @@ function filterInternalQuery(
 ) {
   // this is used to pass query information in rewrites
   // but should not be exposed in final query
-  delete query['nextInternalLocale']
+  query['nextInternalLocale'] = undefined
 
   for (const key in query) {
     const isNextQueryPrefix =
@@ -49,7 +49,7 @@ function filterInternalQuery(
       isNextInterceptionMarkerPrefix ||
       paramKeys.includes(key)
     ) {
-      delete query[key]
+      query[key] = undefined
     }
   }
 }
@@ -398,7 +398,7 @@ export function getServerUtils({
   ) {
     // this is used to pass query information in rewrites
     // but should not be exposed in final query
-    delete query['nextInternalLocale']
+    query['nextInternalLocale'] = undefined
 
     for (const [key, value] of Object.entries(query)) {
       const normalizedKey = normalizeNextQueryParam(key)