Push vs. pull async generators

async-generators.js

async function* foo() {
  console.log("1");
  await Q.delay(1000);
  console.log("2");
  yield "a";
  console.log("3");
  await Q.delay(1000);
  console.log("4");
  yield "b";
}

// If the return type is an "observable" (push-based), once the observable is subscribed to,
// the function will run at the pace that it sets (i.e. the last line will always run 2000 ms
// after the first line)

console.log("before call");
var observable = foo();

console.log("after call; before subscribe");

foo.subscribe(
  function onNext(value) {
    console.log("onNext: ", value);
  },
  function onComplete() {
    console.log("onComplete");
  }
);

// Resulting timeline:
// "before call"
// "after call; before subscribe"
// "1"
// ... wait 1000 ms ...
// "2"
// "onNext: a"
// "3"
// ... wait 1000 ms ...
// "4"
// "onNext: b"
// "onComplete"

// If the return type is an "async generator", i.e. something with { next, throw }
// methods where next returns a promise, then the async generator function can be suspended
// at the whim of the consumer (just like normal sync generator functions). In this example,
// we make the last line of the async generator function run 4000 ms after the first line.

console.log("before call");
var ag = foo();
console.log("after call; before next");
ag.next().then(v => console.log("next1: ", v));

// Consumer can choose not to execute function body until later:

Q.delay(2000).then(() => {
  ag.next().then(v => console.log("next2: ", v));
  ag.next().then(v => console.log("next3: ", v));
});

// Resulting timeline:
// "before call"
// "after call; before next"
// "1"
// ... wait 1000 ms ...
// "2"
// "next1: { value: 'a', done: false }"
// ... wait 2000 ms ...
// "3"
// ... wait 1000 ms ...
// "4"
// "next2: { value: 'b', done: false }"
// "next3: { value: undefined, done: true }"

if next() returns a promise, won't you have to "await" on lines 51, 56-57, to get that ordering? i would otherwise expect the delay(2000) to be concurrent with the generator pausing.

what is Q ? thanks