fixup! feat(core): Export a reusable function to add tracing headers

Author: JPeer264

packages/core/src/fetch.ts

@@ -240,7 +240,7 @@ export function getTracingHeadersForFetchRequest(
     }
 
     return newHeaders;
-  } else if (Array.isArray(originalHeaders)) {
+  } else if (isHeadersInitTupleArray(originalHeaders)) {
     const newHeaders = [...originalHeaders];
 
     if (!originalHeaders.find(header => header[0] === 'sentry-trace')) {
@@ -261,7 +261,7 @@ export function getTracingHeadersForFetchRequest(
       newHeaders.push(['baggage', baggage]);
     }
 
-    return newHeaders as PolymorphicRequestHeaders;
+    return newHeaders;
   } else {
     const existingSentryTraceHeader = 'sentry-trace' in originalHeaders ? originalHeaders['sentry-trace'] : undefined;
     const existingTraceparentHeader = 'traceparent' in originalHeaders ? originalHeaders.traceparent : undefined;
@@ -327,6 +327,18 @@ function isHeaders(headers: unknown): headers is Headers {
   return typeof Headers !== 'undefined' && isInstanceOf(headers, Headers);
 }
 
+/** `HeadersInit` array form: each entry is a [name, value] pair of strings. */
+function isHeadersInitTupleArray(headers: unknown): headers is [string, string][] {
+  if (!Array.isArray(headers)) {
+    return false;
+  }
+
+  return headers.every(
+    (item): item is [string, string] =>
+      Array.isArray(item) && item.length === 2 && typeof item[0] === 'string' && typeof item[1] === 'string',
+  )
+}
+
 function getSpanStartOptions(
   url: string,
   method: string,

packages/core/test/lib/fetch.test.ts

@@ -92,6 +92,21 @@ describe('getTracingHeadersForFetchRequest', () => {
           ['baggage', DEFAULT_BAGGAGE],
         ]);
       });
+
+      it('treats array with non-tuple items as headers object', () => {
+        const returnedHeaders = getTracingHeadersForFetchRequest('/api/test', {
+          headers: ['not-a-tuple', 'also-not-a-tuple'],
+        });
+
+        // Falls through to the else branch (headers object handling)
+        // since the array items are not [string, string] tuples
+        expect(returnedHeaders).toEqual({
+          '0': 'not-a-tuple',
+          '1': 'also-not-a-tuple',
+          'sentry-trace': DEFAULT_SENTRY_TRACE,
+          baggage: DEFAULT_BAGGAGE,
+        });
+      });
     });
 
     describe('and 3rd party baggage header is set', () => {