fix(utils): Better native fetch detection via iframes (#2103)

Fixes #1601

Author: BYK

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - [integrations] fix: Tracing integration fetch headers bug.
 - [node] fix: Force agent-base to be at version 4.3.0 to fix various issues. Fix #1762, fix #2085
+- [utils] fix: Better native `fetch` detection via iframes. Fix #1601
 
 ## 5.4.0
 

packages/utils/src/supports.ts

@@ -1,4 +1,5 @@
 import { getGlobalObject } from './misc';
+import { logger } from './logger';
 
 /**
  * Tells whether current environment supports ErrorEvent objects
@@ -80,14 +81,38 @@ export function supportsFetch(): boolean {
  * Tells whether current environment supports Fetch API natively
  * {@link supportsNativeFetch}.
  *
- * @returns Answer to the given question.
+ * @returns true if `window.fetch` is natively implemented, false otherwise
  */
 export function supportsNativeFetch(): boolean {
   if (!supportsFetch()) {
     return false;
   }
+
+  const isNativeFunc = (func: Function) => func.toString().indexOf('native') !== -1;
   const global = getGlobalObject<Window>();
-  return global.fetch.toString().indexOf('native') !== -1;
+  let result = null;
+  const doc = global.document;
+  if (doc) {
+    const sandbox = doc.createElement('iframe');
+    sandbox.hidden = true;
+    try {
+      doc.head.appendChild(sandbox);
+      if (sandbox.contentWindow && sandbox.contentWindow.fetch) {
+        // tslint:disable-next-line no-unbound-method
+        result = isNativeFunc(sandbox.contentWindow.fetch);
+      }
+      doc.head.removeChild(sandbox);
+    } catch (err) {
+      logger.warn('Could not create sandbox iframe for pure fetch check, bailing to window.fetch: ', err);
+    }
+  }
+
+  if (result === null) {
+    // tslint:disable-next-line no-unbound-method
+    result = isNativeFunc(global.fetch);
+  }
+
+  return result;
 }
 
 /**