mapAsyncIterator: allow 'rejectCallback' to only be synchronous (#3012)

src/subscription/__tests__/mapAsyncIterator-test.js

@@ -354,13 +354,19 @@ describe('mapAsyncIterator', () => {
     );
   });
 
-  async function testClosesSourceWithRejectMapper<T>(mapper: (Error) => T) {
+  it('closes source if mapper throws an error', async () => {
     async function* source() {
       yield 1;
       throw new Error(2);
     }
 
-    const throwOver1 = mapAsyncIterator(source(), (x) => x, mapper);
+    const throwOver1 = mapAsyncIterator(
+      source(),
+      (x) => x,
+      (error) => {
+        throw new Error('Cannot count to ' + error.message);
+      },
+    );
 
     expect(await throwOver1.next()).to.deep.equal({ value: 1, done: false });
 
@@ -379,17 +385,5 @@ describe('mapAsyncIterator', () => {
       value: undefined,
       done: true,
     });
-  }
-
-  it('closes source if mapper throws an error', async () => {
-    await testClosesSourceWithRejectMapper((error) => {
-      throw new Error('Cannot count to ' + error.message);
-    });
-  });
-
-  it('closes source if mapper rejects', async () => {
-    await testClosesSourceWithRejectMapper((error) =>
-      Promise.reject(new Error('Cannot count to ' + error.message)),
-    );
   });
 });

src/subscription/mapAsyncIterator.js

@@ -7,7 +7,7 @@ import type { PromiseOrValue } from '../jsutils/PromiseOrValue';
 export function mapAsyncIterator<T, U>(
   iterable: AsyncIterable<T> | AsyncGenerator<T, void, void>,
   callback: (T) => PromiseOrValue<U>,
-  rejectCallback: (any) => PromiseOrValue<U> = (error) => {
+  rejectCallback: (any) => U = (error) => {
     throw error;
   },
 ): AsyncGenerator<U, void, void> {
@@ -31,10 +31,11 @@ export function mapAsyncIterator<T, U>(
   }
 
   function mapReject(error: mixed) {
-    return asyncMapValue(error, rejectCallback).then(
-      iteratorResult,
-      abruptClose,
-    );
+    try {
+      return { value: rejectCallback(error), done: false };
+    } catch (callbackError) {
+      return abruptClose(callbackError);
+    }
   }
 
   /* TODO: Flow doesn't support symbols as keys: