fix(browser): Handle case where fetch can be undefined (#5973)

Some users reported cases where the `nativeFetch` call was returning undefined. I have no idea how this happens, but in order for us to not send unnecessary errors to Sentry, let's be a little defensive in the usage of fetch.

packages/browser/src/transports/fetch.ts

@@ -1,8 +1,9 @@
 import { createTransport } from '@sentry/core';
 import { Transport, TransportMakeRequestResponse, TransportRequest } from '@sentry/types';
+import { rejectedSyncPromise } from '@sentry/utils';
 
 import { BrowserTransportOptions } from './types';
-import { FetchImpl, getNativeFetchImplementation } from './utils';
+import { clearCachedFetchImplementation, FetchImpl, getNativeFetchImplementation } from './utils';
 
 /**
  * Creates a Transport that uses the Fetch API to send events to Sentry.
@@ -30,13 +31,18 @@ export function makeFetchTransport(
       ...options.fetchOptions,
     };
 
-    return nativeFetch(options.url, requestOptions).then(response => ({
-      statusCode: response.status,
-      headers: {
-        'x-sentry-rate-limits': response.headers.get('X-Sentry-Rate-Limits'),
-        'retry-after': response.headers.get('Retry-After'),
-      },
-    }));
+    try {
+      return nativeFetch(options.url, requestOptions).then(response => ({
+        statusCode: response.status,
+        headers: {
+          'x-sentry-rate-limits': response.headers.get('X-Sentry-Rate-Limits'),
+          'retry-after': response.headers.get('Retry-After'),
+        },
+      }));
+    } catch (e) {
+      clearCachedFetchImplementation();
+      return rejectedSyncPromise(e);
+    }
   }
 
   return createTransport(options, makeRequest);

packages/browser/src/transports/utils.ts

@@ -2,7 +2,7 @@ import { isNativeFetch, logger } from '@sentry/utils';
 
 import { WINDOW } from '../helpers';
 
-let cachedFetchImpl: FetchImpl;
+let cachedFetchImpl: FetchImpl | undefined = undefined;
 
 export type FetchImpl = typeof fetch;
 
@@ -78,3 +78,8 @@ export function getNativeFetchImplementation(): FetchImpl {
   return (cachedFetchImpl = fetchImpl.bind(WINDOW));
   /* eslint-enable @typescript-eslint/unbound-method */
 }
+
+/** Clears cached fetch impl */
+export function clearCachedFetchImplementation(): void {
+  cachedFetchImpl = undefined;
+}

packages/browser/test/unit/transports/fetch.test.ts

@@ -98,4 +98,13 @@ describe('NewFetchTransport', () => {
       ...REQUEST_OPTIONS,
     });
   });
+
+  it('handles when `getNativeFetchImplementation` is undefined', async () => {
+    const mockFetch = jest.fn(() => undefined) as unknown as FetchImpl;
+    const transport = makeFetchTransport(DEFAULT_FETCH_TRANSPORT_OPTIONS, mockFetch);
+
+    expect(mockFetch).toHaveBeenCalledTimes(0);
+    await expect(() => transport.send(ERROR_ENVELOPE)).not.toThrow();
+    expect(mockFetch).toHaveBeenCalledTimes(1);
+  });
 });