Ensure onClose is only called once we receive a value from the listener

src/index.js

@@ -21,10 +21,14 @@ function callbackToAsyncIterator<CallbackInput: any, ReturnVal: any>(
     let pushQueue = [];
     let listening = true;
     let listenerReturnValue;
+    let listenerReturnedValue = false;
+    let closingWaitingOnListenerReturnValue = false;
     // Start listener
     listener(value => pushValue(value))
       .then(a => {
         listenerReturnValue = a;
+        listenerReturnedValue = true;
+        if (closingWaitingOnListenerReturnValue) emptyQueue();
       })
       .catch(err => {
         onError(err);
@@ -49,6 +53,10 @@ function callbackToAsyncIterator<CallbackInput: any, ReturnVal: any>(
     }
 
     function emptyQueue() {
+      if (onClose && !listenerReturnedValue) {
+        closingWaitingOnListenerReturnValue = true;
+        return;
+      }
       if (listening) {
         listening = false;
         pullQueue.forEach(resolve => resolve({ value: undefined, done: true }));

src/test/index.test.js

@@ -145,6 +145,24 @@ describe('options', () => {
     await iter.return();
   });
 
+  it('should call onClose with the return value from an listener only after the promise resolves', async () => {
+    const returnValue = 'asdf';
+    const listener = (cb: () => void) =>
+      new Promise(res => {
+        res(returnValue);
+      });
+
+    expect.hasAssertions();
+    const iter = asyncify(listener, {
+      onClose: val => {
+        expect(val).toEqual(returnValue);
+      },
+    });
+    // Wait a tick so that the promise resolves with the return value
+    iter.return();
+    await new Promise(res => setTimeout(res, 10));
+  });
+
   describe('buffering', () => {
     it('should not buffer incoming values if disabled', async () => {
       const listener = (cb: (arg: number) => void) =>