ref: Add fast-path to fetchImpl and cleanup redundant iframe (#3341)

* ref: Add fast-path to fetchImpl

* fix: Cleanup after fetch impl detection

Author: kamilogorek

packages/browser/src/transports/fetch.ts

@@ -1,6 +1,6 @@
 import { eventToSentryRequest, sessionToSentryRequest } from '@sentry/core';
 import { Event, Response, SentryRequest, Session, TransportOptions } from '@sentry/types';
-import { getGlobalObject, logger, supportsReferrerPolicy, SyncPromise } from '@sentry/utils';
+import { getGlobalObject, isNativeFetch, logger, supportsReferrerPolicy, SyncPromise } from '@sentry/utils';
 
 import { BaseTransport } from './base';
 
@@ -45,24 +45,33 @@ type FetchImpl = typeof fetch;
  * Safari:  resource blocked by content blocker
  */
 function getNativeFetchImplementation(): FetchImpl {
-  // Make sure that the fetch we use is always the native one.
+  /* eslint-disable @typescript-eslint/unbound-method */
+
+  // Fast path to avoid DOM I/O
   const global = getGlobalObject<Window>();
+  if (isNativeFetch(global.fetch)) {
+    return global.fetch.bind(global);
+  }
+
   const document = global.document;
+  let fetchImpl = global.fetch;
   // eslint-disable-next-line deprecation/deprecation
   if (typeof document?.createElement === `function`) {
     try {
       const sandbox = document.createElement('iframe');
       sandbox.hidden = true;
       document.head.appendChild(sandbox);
       if (sandbox.contentWindow?.fetch) {
-        return sandbox.contentWindow.fetch.bind(global);
+        fetchImpl = sandbox.contentWindow.fetch;
       }
       document.head.removeChild(sandbox);
     } catch (e) {
       logger.warn('Could not create sandbox iframe for pure fetch check, bailing to window.fetch: ', e);
     }
   }
-  return global.fetch.bind(global);
+
+  return fetchImpl.bind(global);
+  /* eslint-enable @typescript-eslint/unbound-method */
 }
 
 /** `fetch` based transport */

packages/utils/src/supports.ts

@@ -73,7 +73,7 @@ export function supportsFetch(): boolean {
  * isNativeFetch checks if the given function is a native implementation of fetch()
  */
 // eslint-disable-next-line @typescript-eslint/ban-types
-function isNativeFetch(func: Function): boolean {
+export function isNativeFetch(func: Function): boolean {
   return func && /^function fetch\(\)\s+\{\s+\[native code\]\s+\}$/.test(func.toString());
 }