Making an asynchronous generator *[Symbol.asyncIterator] without async/await but with promises only

Question

Now this code works just fine

async *[Symbol.asyncIterator](){
  var promise;
  while (true){
    promise = this.#HEAD.promise;
    this.size--;
    this.#HEAD.next ? this.#HEAD = this.#HEAD.next
                    : this.#LAST = void 0;
    yield await promise;
  };
};

Say if i don't want to use the async / await abstraction then how can i implement the same functionality only with promises?

I naively tried

*[Symbol.asyncIterator](){
  var promise;
  while (true){
    promise = this.#HEAD.promise;
    this.size--;
    this.#HEAD.next ? this.#HEAD = this.#HEAD.next
                    : this.#LAST = void 0;
    promise.then(yield);
  };
};

but it returns undefined; presumingly yield not being a function. I checked out this question but it's not about generators and no yield is involved. Is there a way to implement this?

Edit: yield await promise in an async generator seems to be wasteful. Use yield promise instead. Check the comments under T.J. Crowders answer.

Answer 1

An async iterator has to produce promises for result objects (objects in the form {value, done}). You can't do that in a non-async generator by using a promise as yield's operand because the operand you give yield becomes the value in the result object, not the result object itself. That is, when you do:

yield 42;

...in an async generator function, it produces a promise for {value: 42, done: false} (using TypeScript notation, Promise<{value: 42, done: false}>). If you do:

yield somePromise;

...in a non-async generator function, it produces {value: somePromise, done: false} (TS: {value: Promise, done: false}). That's not what an asyncIterator function is defined to return. It has to return a promise for the {value, done} object, not a non-promise object.

So you have at least two choices if you want to avoid using async/await:

1. Define your object such that it isn't async iterable, just iterable, and the values it produces are {value: Promise, done: boolean}.

2. Define it as async iterable and don't use yield. Write the next method explicitly.

I'd definitely go for #2 for the semantics. It's hard to show precisely without more information about your object, but roughly:

[Symbol.asyncIterator](){
    let current = this.#HEAD;
    return {
        next() {
            if (/*done condition*/) {
                return Promise.resolve({done: true});
            }
            return current.promise.then(value => {
                current = current.next; // or whatever
                return {value, done: false};
            });
        }
    };
}

Or if you want the async iterator object to have the standard prototype, put this somewhere where you can reuse it:

const asyncIteratorPrototype =
    Object.getPrototypeOf(
        Object.getPrototypeOf(
            (async function *(){}).prototype
        )
    );

then:

[Symbol.asyncIterator](){
    let current = this.#HEAD;
    return Object.assign(Object.create(asyncIterator), {
        next() {
            if (/*done condition*/) {
                return Promise.resolve({done: true});
            }
            return current.promise.then(value => {
                current = current.next; // or whatever
                return {value, done: false};
            });
        }
    });
}