mapAsyncIterator: move promise resolution into mapResult (#3028)

src/subscription/mapAsyncIterator.js

@@ -23,12 +23,14 @@ export function mapAsyncIterator<T, U>(
     throw error;
   }
 
-  async function mapResult(result: IteratorResult<T, void>) {
-    if (result.done) {
-      return result;
-    }
-
+  async function mapResult(resultPromise: Promise<IteratorResult<T, void>>) {
     try {
+      const result = await resultPromise;
+
+      if (result.done) {
+        return result;
+      }
+
       return { value: await callback(result.value), done: false };
     } catch (callbackError) {
       return abruptClose(callbackError);
@@ -39,16 +41,16 @@ export function mapAsyncIterator<T, U>(
      https://github.com/facebook/flow/issues/3258 */
   return ({
     next(): Promise<IteratorResult<U, void>> {
-      return iterator.next().then(mapResult, abruptClose);
+      return mapResult(iterator.next());
     },
     return() {
       return typeof iterator.return === 'function'
-        ? iterator.return().then(mapResult, abruptClose)
+        ? mapResult(iterator.return())
         : Promise.resolve({ value: undefined, done: true });
     },
     throw(error?: mixed): Promise<IteratorResult<U, void>> {
       if (typeof iterator.throw === 'function') {
-        return iterator.throw(error).then(mapResult, abruptClose);
+        return mapResult(iterator.throw(error));
       }
       return Promise.reject(error).catch(abruptClose);
     },