perf: avoid Headers initialization (#2468)

Author: tsctx

lib/cache/symbols.js

@@ -1,5 +1,5 @@
 'use strict'
 
 module.exports = {
-  kConstruct: Symbol('constructable')
+  kConstruct: require('../core/symbols').kConstruct
 }

lib/core/symbols.js

@@ -58,5 +58,6 @@ module.exports = {
   kHTTP1BuildRequest: Symbol('http1 build request'),
   kHTTP2CopyHeaders: Symbol('http2 copy headers'),
   kHTTPConnVersion: Symbol('http connection version'),
-  kRetryHandlerDefaultRetry: Symbol('retry agent default retry')
+  kRetryHandlerDefaultRetry: Symbol('retry agent default retry'),
+  kConstruct: Symbol('constructable')
 }

lib/fetch/headers.js

@@ -2,7 +2,7 @@
 
 'use strict'
 
-const { kHeadersList } = require('../core/symbols')
+const { kHeadersList, kConstruct } = require('../core/symbols')
 const { kGuard } = require('./symbols')
 const { kEnumerableProperty } = require('../core/util')
 const {
@@ -240,6 +240,9 @@ class HeadersList {
 // https://fetch.spec.whatwg.org/#headers-class
 class Headers {
   constructor (init = undefined) {
+    if (init === kConstruct) {
+      return
+    }
     this[kHeadersList] = new HeadersList()
 
     // The new Headers(init) constructor steps are:

lib/fetch/request.js

@@ -28,13 +28,12 @@ const { kHeaders, kSignal, kState, kGuard, kRealm } = require('./symbols')
 const { webidl } = require('./webidl')
 const { getGlobalOrigin } = require('./global')
 const { URLSerializer } = require('./dataURL')
-const { kHeadersList } = require('../core/symbols')
+const { kHeadersList, kConstruct } = require('../core/symbols')
 const assert = require('assert')
 const { getMaxListeners, setMaxListeners, getEventListeners, defaultMaxListeners } = require('events')
 
 let TransformStream = globalThis.TransformStream
 
-const kInit = Symbol('init')
 const kAbortController = Symbol('abortController')
 
 const requestFinalizer = new FinalizationRegistry(({ signal, abort }) => {
@@ -45,7 +44,7 @@ const requestFinalizer = new FinalizationRegistry(({ signal, abort }) => {
 class Request {
   // https://fetch.spec.whatwg.org/#dom-request
   constructor (input, init = {}) {
-    if (input === kInit) {
+    if (input === kConstruct) {
       return
     }
 
@@ -398,7 +397,7 @@ class Request {
     // 30. Set this’s headers to a new Headers object with this’s relevant
     // Realm, whose header list is request’s header list and guard is
     // "request".
-    this[kHeaders] = new Headers()
+    this[kHeaders] = new Headers(kConstruct)
     this[kHeaders][kHeadersList] = request.headersList
     this[kHeaders][kGuard] = 'request'
     this[kHeaders][kRealm] = this[kRealm]
@@ -725,10 +724,10 @@ class Request {
 
     // 3. Let clonedRequestObject be the result of creating a Request object,
     // given clonedRequest, this’s headers’s guard, and this’s relevant Realm.
-    const clonedRequestObject = new Request(kInit)
+    const clonedRequestObject = new Request(kConstruct)
     clonedRequestObject[kState] = clonedRequest
     clonedRequestObject[kRealm] = this[kRealm]
-    clonedRequestObject[kHeaders] = new Headers()
+    clonedRequestObject[kHeaders] = new Headers(kConstruct)
     clonedRequestObject[kHeaders][kHeadersList] = clonedRequest.headersList
     clonedRequestObject[kHeaders][kGuard] = this[kHeaders][kGuard]
     clonedRequestObject[kHeaders][kRealm] = this[kHeaders][kRealm]

lib/fetch/response.js

@@ -23,7 +23,7 @@ const { webidl } = require('./webidl')
 const { FormData } = require('./formdata')
 const { getGlobalOrigin } = require('./global')
 const { URLSerializer } = require('./dataURL')
-const { kHeadersList } = require('../core/symbols')
+const { kHeadersList, kConstruct } = require('../core/symbols')
 const assert = require('assert')
 const { types } = require('util')
 
@@ -144,7 +144,7 @@ class Response {
     // 2. Set this’s headers to a new Headers object with this’s relevant
     // Realm, whose header list is this’s response’s header list and guard
     // is "response".
-    this[kHeaders] = new Headers()
+    this[kHeaders] = new Headers(kConstruct)
     this[kHeaders][kGuard] = 'response'
     this[kHeaders][kHeadersList] = this[kState].headersList
     this[kHeaders][kRealm] = this[kRealm]