Move 'isAsyncIterable' into 'jsutils' (graphql#2775)

src/jsutils/__tests__/isAsyncIterable-test.js

@@ -0,0 +1,53 @@
+import { expect } from 'chai';
+import { describe, it } from 'mocha';
+
+import identityFunc from '../identityFunc';
+import isAsyncIterable from '../isAsyncIterable';
+
+describe('isAsyncIterable', () => {
+  it('should return `true` for AsyncIterable', () => {
+    // $FlowFixMe[prop-missing] Flow doesn't support Symbol.asyncIterator
+    const asyncIteratable = { [Symbol.asyncIterator]: identityFunc };
+    expect(isAsyncIterable(asyncIteratable)).to.equal(true);
+
+    // istanbul ignore next (Never called and use just as a placeholder)
+    async function* asyncGeneratorFunc() {
+      /* do nothing */
+    }
+
+    expect(isAsyncIterable(asyncGeneratorFunc())).to.equal(true);
+
+    // But async generator function itself is not iteratable
+    expect(isAsyncIterable(asyncGeneratorFunc)).to.equal(false);
+  });
+
+  it('should return `false` for all other values', () => {
+    expect(isAsyncIterable(null)).to.equal(false);
+    expect(isAsyncIterable(undefined)).to.equal(false);
+
+    expect(isAsyncIterable('ABC')).to.equal(false);
+    expect(isAsyncIterable('0')).to.equal(false);
+    expect(isAsyncIterable('')).to.equal(false);
+
+    expect(isAsyncIterable([])).to.equal(false);
+    expect(isAsyncIterable(new Int8Array(1))).to.equal(false);
+
+    expect(isAsyncIterable({})).to.equal(false);
+    expect(isAsyncIterable({ iterable: true })).to.equal(false);
+
+    const iterator = { [Symbol.iterator]: identityFunc };
+    expect(isAsyncIterable(iterator)).to.equal(false);
+
+    // istanbul ignore next (Never called and use just as a placeholder)
+    function* generatorFunc() {
+      /* do nothing */
+    }
+    expect(isAsyncIterable(generatorFunc())).to.equal(false);
+
+    const invalidAsyncIteratable = {
+      // $FlowFixMe[prop-missing] Flow doesn't support Symbol.asyncIterator
+      [Symbol.asyncIterator]: { next: identityFunc },
+    };
+    expect(isAsyncIterable(invalidAsyncIteratable)).to.equal(false);
+  });
+});

src/jsutils/isAsyncIterable.js

@@ -0,0 +1,17 @@
+import { SYMBOL_ASYNC_ITERATOR } from '../polyfills/symbols';
+
+/**
+ * Returns true if the provided object implements the AsyncIterator protocol via
+ * either implementing a `Symbol.asyncIterator` or `"@@asyncIterator"` method.
+ */
+declare function isAsyncIterable(value: mixed): boolean %checks(value instanceof
+  AsyncIterable);
+
+// eslint-disable-next-line no-redeclare
+export default function isAsyncIterable(maybeAsyncIterable) {
+  if (maybeAsyncIterable == null || typeof maybeAsyncIterable !== 'object') {
+    return false;
+  }
+
+  return typeof maybeAsyncIterable[SYMBOL_ASYNC_ITERATOR] === 'function';
+}

src/subscription/subscribe.js

@@ -1,6 +1,5 @@
-import { SYMBOL_ASYNC_ITERATOR } from '../polyfills/symbols';
-
 import inspect from '../jsutils/inspect';
+import isAsyncIterable from '../jsutils/isAsyncIterable';
 import { addPath, pathToArray } from '../jsutils/Path';
 
 import { GraphQLError } from '../error/GraphQLError';
@@ -158,7 +157,7 @@ function subscribeImpl(
     // Note: Flow can't refine isAsyncIterable, so explicit casts are used.
     isAsyncIterable(resultOrStream)
       ? mapAsyncIterator(
-          ((resultOrStream: any): AsyncIterable<mixed>),
+          resultOrStream,
           mapSourceToResponse,
           reportGraphQLError,
         )
@@ -289,24 +288,10 @@ function executeSubscription(
             `Received: ${inspect(eventStream)}.`,
         );
       }
-
-      // Note: isAsyncIterable above ensures this will be correct.
-      return ((eventStream: any): AsyncIterable<mixed>);
+      return eventStream;
     },
     (error) => {
       throw locatedError(error, fieldNodes, pathToArray(path));
     },
   );
 }
-
-/**
- * Returns true if the provided object implements the AsyncIterator protocol via
- * either implementing a `Symbol.asyncIterator` or `"@@asyncIterator"` method.
- */
-function isAsyncIterable(maybeAsyncIterable: mixed): boolean {
-  if (maybeAsyncIterable == null || typeof maybeAsyncIterable !== 'object') {
-    return false;
-  }
-
-  return typeof maybeAsyncIterable[SYMBOL_ASYNC_ITERATOR] === 'function';
-}