Don't override the global value

Author: neilalexander

src/net/http/roundtrip_js.go

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build js && wasm
 // +build js,wasm
 
 package http
@@ -41,23 +40,19 @@ const jsFetchCreds = "js.fetch:credentials"
 // Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
 const jsFetchRedirect = "js.fetch:redirect"
 
-var useFakeNetwork = js.Global().Get("fetch").IsUndefined()
+// jsFetchMissing will be true if the Fetch API is not present in
+// the browser globals.
+var jsFetchMissing = !js.Global().Get("fetch").IsUndefined()
 
 // RoundTrip implements the RoundTripper interface using the WHATWG Fetch API.
 func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 	// The Transport has a documented contract that states that if the DialContext or
 	// DialTLSContext functions are set, they will be used to set up the connections.
 	// If they aren't set then the documented contract is to use Dial or DialTLS, even
 	// though they are deprecated. Therefore, if any of these are set, we should obey
-	// the contract and dial using the regular round-trip instead. Otherwise we will
-	// end up calling the browser Fetch API unexpectedly.
-	if t.Dial != nil || t.DialContext != nil || t.DialTLS != nil || t.DialTLSContext != nil {
-		useFakeNetwork = true
-	}
-
-	// If the browser Fetch API is unavailable, or the above conditions are met, then
-	// we will lean on fake networking to set up the connection.
-	if useFakeNetwork {
+	// the contract and dial using the regular round-trip instead. Otherwise, we'll try
+	// to fall back on the Fetch API, unless it's not available.
+	if t.Dial != nil || t.DialContext != nil || t.DialTLS != nil || t.DialTLSContext != nil || jsFetchMissing {
 		return t.roundTrip(req)
 	}