Wait for an async operation inside asyncIterator generator function

Question

I have this Queue class (not the real implementation, but it exemplifies my point):

class Queue {
    constructor() {
        this._arr = [];
    }

    async push(elem) {
        this._arr.push(elem);
    }

    async pop() {
        return this._arr.pop();
    }

    *[Symbol.asyncIterator]() {
        do {
            let res = await this.pop(); // here is the problem
            if (res) yield res;
        } while (res);
    }
}

It's just a wrapper for a Javascript Array with the exception that its methods return a Promise.

What I want to do is to yield conditionally based on the return value of the pop() method, which I cannot do because await is not a valid operation inside an asyncIterator generator function.

I thought about having a flag which is set in the previous iteration:

*[Symbol.asyncIterator]() {
    let continue = true;
    do {
        yield this.pop().then(v => {
            if (!v) continue = false;
            return v
        });
    } while (continue);
}

But this would still return a undefined value in the last execution of pop().

I could handle this in the calling code by checking for a undefined value as a signal of the end of the iteration, but I was wondering if there was a better approach at tackling this problem.

For a real queue implementation maybe have a look at this. It also implements [Symbol.asyncIterator]() fittingly, without any generator function. — Bergi
– Bergi, Commented Apr 9, 2021 at 1:41

Bergi · Accepted Answer · 2021-04-09 01:45:32Z

1

You can use an async generator function (MDN docs missing, but see e.g. this article) for the implementation of the [Symbol.asyncIterator]() method:

async *[Symbol.asyncIterator]() { /*
^^^^^ */
    while (this._arr.length) {
        yield await this.pop(); // no longer a problem
    }
}

edited Apr 9, 2021 at 1:45

answered Apr 9, 2021 at 1:39

Bergi

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1 Comment

Federico Over a year ago

Ahh, the simplicity. I knew something like this existed somewhere. Thanks!

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