Fix Flow issues with Symbol.asyncIterator

Author: IvanGoncharov

flow-typed/core.js

@@ -260,7 +260,7 @@ declare opaque type $SymbolToStringTag: symbol;
 declare opaque type $SymbolUnscopables: symbol;
 
 declare class Symbol {
-  static asyncIterator: string; // graphql-js HACK
+  static asyncIterator: '@@asyncIterator'; // graphql-js HACK
 
   /**
    * Returns a new unique Symbol value.
@@ -287,7 +287,7 @@ declare class Symbol {
    * by Array.prototype.concat.
    */
   static isConcatSpreadable: $SymboIsConcatSpreadable;
-  static iterator: string; // polyfill '@@iterator'
+  static iterator: '@@iterator';
   /**
    * Returns a key from the global symbol registry matching the given Symbol if found.
    * Otherwise, returns a undefined.

src/subscription/__tests__/mapAsyncIterator-test.js

@@ -25,7 +25,7 @@ describe('mapAsyncIterator', () => {
   it('maps over async iterable', async () => {
     const items = [1, 2, 3];
 
-    const iterable: $FlowFixMe = {
+    const iterable = {
       [Symbol.asyncIterator]() {
         return this;
       },
@@ -122,7 +122,7 @@ describe('mapAsyncIterator', () => {
   it('allows returning early from mapped async iterable', async () => {
     const items = [1, 2, 3];
 
-    const iterable: any = {
+    const iterable = {
       [Symbol.asyncIterator]() {
         return this;
       },
@@ -185,7 +185,7 @@ describe('mapAsyncIterator', () => {
   it('allows throwing errors through async iterable', async () => {
     const items = [1, 2, 3];
 
-    const iterable: any = {
+    const iterable = {
       [Symbol.asyncIterator]() {
         return this;
       },

src/subscription/__tests__/simplePubSub-test.js

@@ -6,7 +6,7 @@ import { SimplePubSub } from './simplePubSub';
 describe('SimplePubSub', () => {
   it('subscribe async-iterator mock', async () => {
     const pubsub = new SimplePubSub();
-    const iterator = pubsub.getSubscriber();
+    const iterator = pubsub.getSubscriber((x) => x);
 
     // Queue up publishes
     expect(pubsub.emit('Apple')).to.equal(true);

src/subscription/__tests__/simplePubSub.js

@@ -16,8 +16,8 @@ export class SimplePubSub<T> {
     return this._subscribers.size > 0;
   }
 
-  getSubscriber<R>(transform?: (T) => R): AsyncGenerator<R, void, void> {
-    const pullQueue = [];
+  getSubscriber<R>(transform: (T) => R): AsyncGenerator<R, void, void> {
+    const pullQueue: Array<(result: IteratorResult<R, void>) => void> = [];
     const pushQueue = [];
     let listening = true;
     this._subscribers.add(pushValue);
@@ -32,9 +32,7 @@ export class SimplePubSub<T> {
       pushQueue.length = 0;
     };
 
-    /* TODO: Flow doesn't support symbols as keys:
-       https://github.com/facebook/flow/issues/3258 */
-    return ({
+    return {
       next() {
         if (!listening) {
           return Promise.resolve({ value: undefined, done: true });
@@ -45,7 +43,7 @@ export class SimplePubSub<T> {
         }
         return new Promise((resolve) => pullQueue.push(resolve));
       },
-      return() {
+      return(): Promise<IteratorResult<R, void>> {
         emptyQueue();
         return Promise.resolve({ value: undefined, done: true });
       },
@@ -56,10 +54,10 @@ export class SimplePubSub<T> {
       [Symbol.asyncIterator]() {
         return this;
       },
-    }: any);
+    };
 
     function pushValue(event: T): void {
-      const value = transform != null ? transform(event) : event;
+      const value: R = transform(event);
       if (pullQueue.length > 0) {
         pullQueue.shift()({ value, done: false });
       } else {

src/subscription/__tests__/subscribe-test.js

@@ -187,7 +187,7 @@ describe('Subscription Initialization Phase', () => {
         fields: {
           importantEmail: {
             type: GraphQLString,
-            subscribe: () => pubsub.getSubscriber(),
+            subscribe: () => pubsub.getSubscriber((x) => x),
           },
         },
       }),
@@ -219,7 +219,7 @@ describe('Subscription Initialization Phase', () => {
             type: GraphQLString,
             subscribe: async () => {
               await resolveOnNextTick();
-              return pubsub.getSubscriber();
+              return pubsub.getSubscriber((x) => x);
             },
           },
         },
@@ -253,15 +253,15 @@ describe('Subscription Initialization Phase', () => {
           type: EmailEventType,
           subscribe() {
             didResolveImportantEmail = true;
-            return new SimplePubSub().getSubscriber();
+            return new SimplePubSub().getSubscriber((x) => x);
           },
         },
         nonImportantEmail: {
           type: EmailEventType,
           // istanbul ignore next (Shouldn't be called)
           subscribe() {
             didResolveNonImportantEmail = true;
-            return new SimplePubSub().getSubscriber();
+            return new SimplePubSub().getSubscriber((x) => x);
           },
         },
       },

src/subscription/mapAsyncIterator.js

@@ -8,9 +8,8 @@ export function mapAsyncIterator<T, U>(
   iterable: AsyncIterable<T> | AsyncGenerator<T, void, void>,
   callback: (T) => PromiseOrValue<U>,
 ): AsyncGenerator<U, void, void> {
-  // $FlowFixMe[prop-missing]
-  const iteratorMethod = iterable[Symbol.asyncIterator];
-  const iterator: any = iteratorMethod.call(iterable);
+  // $FlowIssue[incompatible-use]
+  const iterator = iterable[Symbol.asyncIterator]();
 
   async function abruptClose(error: mixed) {
     if (typeof iterator.return === 'function') {
@@ -37,13 +36,11 @@ export function mapAsyncIterator<T, U>(
     }
   }
 
-  /* TODO: Flow doesn't support symbols as keys:
-     https://github.com/facebook/flow/issues/3258 */
-  return ({
+  return {
     next(): Promise<IteratorResult<U, void>> {
       return mapResult(iterator.next());
     },
-    return() {
+    return(): Promise<IteratorResult<U, void>> {
       return typeof iterator.return === 'function'
         ? mapResult(iterator.return())
         : Promise.resolve({ value: undefined, done: true });
@@ -57,5 +54,5 @@ export function mapAsyncIterator<T, U>(
     [Symbol.asyncIterator]() {
       return this;
     },
-  }: $FlowFixMe);
+  };
 }