Polyfill Symbol.asyncIterator everywhere

src/lib/abstract-ops/ecmascript.ts

@@ -110,6 +110,12 @@ export function CreateAsyncFromSyncIterator<T>(syncIteratorRecord: SyncIteratorR
   return { iterator: asyncIterator, nextMethod, done: false };
 }
 
+// Aligns with core-js/modules/es.symbol.async-iterator.js
+export const SymbolAsyncIterator: (typeof Symbol)['asyncIterator'] =
+  Symbol.asyncIterator ??
+  Symbol.for?.('Symbol.asyncIterator') ??
+  '@@asyncIterator';
+
 export type SyncOrAsyncIterable<T> = Iterable<T> | AsyncIterable<T>;
 export type SyncOrAsyncIteratorMethod<T> = () => (Iterator<T> | AsyncIterator<T>);
 
@@ -131,7 +137,7 @@ function GetIterator<T>(
   assert(hint === 'sync' || hint === 'async');
   if (method === undefined) {
     if (hint === 'async') {
-      method = GetMethod(obj as AsyncIterable<T>, Symbol.asyncIterator);
+      method = GetMethod(obj as AsyncIterable<T>, SymbolAsyncIterator);
       if (method === undefined) {
         const syncMethod = GetMethod(obj as Iterable<T>, Symbol.iterator);
         const syncIteratorRecord = GetIterator(obj as Iterable<T>, 'sync', syncMethod);

src/lib/readable-stream.ts

@@ -49,7 +49,7 @@ import type {
 } from './readable-stream/underlying-source';
 import { noop } from '../utils';
 import { setFunctionName, typeIsObject } from './helpers/miscellaneous';
-import { CreateArrayFromList } from './abstract-ops/ecmascript';
+import { CreateArrayFromList, SymbolAsyncIterator } from './abstract-ops/ecmascript';
 import { CancelSteps } from './abstract-ops/internal-methods';
 import { IsNonNegativeNumber } from './abstract-ops/miscellaneous';
 import { assertObject, assertRequiredArgument } from './validators/basic';
@@ -80,12 +80,6 @@ export type ReadableByteStream = ReadableStream<NonShared<Uint8Array>> & {
 
 type ReadableStreamState = 'readable' | 'closed' | 'errored';
 
-// Aligns with core-js/modules/es.symbol.async-iterator.js
-const SymbolAsyncIterator: (typeof Symbol)['asyncIterator'] =
-  Symbol.asyncIterator ??
-  Symbol.for?.('Symbol.asyncIterator') ??
-  '@@asyncIterator';
-
 /**
  * A readable stream represents a source of data, from which you can read.
  *

src/lib/readable-stream/async-iterator.ts

@@ -140,9 +140,7 @@ const ReadableStreamAsyncIteratorPrototype: ReadableStreamAsyncIteratorInstance<
     return this._asyncIteratorImpl.return(value);
   }
 } as any;
-if (AsyncIteratorPrototype !== undefined) {
-  Object.setPrototypeOf(ReadableStreamAsyncIteratorPrototype, AsyncIteratorPrototype);
-}
+Object.setPrototypeOf(ReadableStreamAsyncIteratorPrototype, AsyncIteratorPrototype);
 
 // Abstract operations for the ReadableStream.
 

src/target/es2018/stub/async-iterator-prototype.ts

@@ -1,5 +1,5 @@
 /// <reference lib="es2018.asynciterable" />
 
 /* eslint-disable @typescript-eslint/no-empty-function */
-export const AsyncIteratorPrototype: AsyncIterable<any> | undefined =
+export const AsyncIteratorPrototype: AsyncIterable<any> =
   Object.getPrototypeOf(Object.getPrototypeOf(async function* (): AsyncIterableIterator<any> {}).prototype);

src/target/es5/stub/async-iterator-prototype.ts

@@ -1,16 +1,13 @@
 /// <reference lib="es2018.asynciterable" />
 
-export let AsyncIteratorPrototype: AsyncIterable<any> | undefined;
+import { SymbolAsyncIterator } from '../../../lib/abstract-ops/ecmascript';
 
-if (typeof Symbol.asyncIterator === 'symbol') {
-  // We're running inside a ES2018+ environment, but we're compiling to an older syntax.
-  // We cannot access %AsyncIteratorPrototype% without non-ES2018 syntax, but we can re-create it.
-  AsyncIteratorPrototype = {
-    // 25.1.3.1 %AsyncIteratorPrototype% [ @@asyncIterator ] ( )
-    // https://tc39.github.io/ecma262/#sec-asynciteratorprototype-asynciterator
-    [Symbol.asyncIterator](this: AsyncIterator<any>) {
-      return this;
-    }
-  };
-  Object.defineProperty(AsyncIteratorPrototype, Symbol.asyncIterator, { enumerable: false });
-}
+// We cannot access %AsyncIteratorPrototype% without non-ES2018 syntax, but we can re-create it.
+export const AsyncIteratorPrototype: AsyncIterable<any> = {
+  // 25.1.3.1 %AsyncIteratorPrototype% [ @@asyncIterator ] ( )
+  // https://tc39.github.io/ecma262/#sec-asynciteratorprototype-asynciterator
+  [SymbolAsyncIterator](this: AsyncIterator<any>) {
+    return this;
+  }
+};
+Object.defineProperty(AsyncIteratorPrototype, SymbolAsyncIterator, { enumerable: false });