AsyncQueue.js

/**
 * An asynchronously iterable queue class. Add values with enqueue()
 * and remove them with dequeue(). dequeue() returns a Promise, which
 * means that values can be dequeued before they are enqueued. The
 * class implements [Symbol.asyncIterator] and next() so that it can
 * be used with the for/await loop (which will not terminate until
 * the close() method is called.)
 */
class AsyncQueue {
    constructor() {
        // Values that have been queued but not dequeued yet are stored here
        this.values = [];
        // When Promises are dequeued before their corresponding values are
        // queued, the resolve methods for those Promises are stored here.
        this.resolvers = [];
        // Once closed, no more values can be enqueued, and no more unfulfilled
        // Promises returned.
        this.closed = false;
    }

    enqueue(value) {
        if (this.closed) {
            throw new Error("AsyncQueue closed");
        }
        if (this.resolvers.length > 0) {
            // If this value has already been promised, resolve that Promise
            const resolve = this.resolvers.shift();
            resolve(value);
        }
        else {
            // Otherwise, queue it up
            this.values.push(value);
        }
    }

    dequeue() {
        if (this.values.length > 0) {
            // If there is a queued value, return a resolved Promise for it
            const value = this.values.shift();
            return Promise.resolve(value);
        }
        else if (this.closed) {
            // If no queued values and we're closed, return a resolved
            // Promise for the "end-of-stream" marker
            return Promise.resolve(AsyncQueue.EOS);
        }
        else {
            // Otherwise, return an unresolved Promise,
            // queuing the resolver function for later use
            return new Promise((resolve) => { this.resolvers.push(resolve); });
        }
    }

    close() {
        // Once the queue is closed, no more values will be enqueued.
        // So resolve any pending Promises with the end-of-stream marker
        while(this.resolvers.length > 0) {
            this.resolvers.shift()(AsyncQueue.EOS);
        }
        this.closed = true;
    }

    // Define the method that makes this class asynchronously iterable
    [Symbol.asyncIterator]() { return this; }

    // Define the method that makes this an asynchronous iterator. The
    // dequeue() Promise resolves to a value or the EOS sentinel if we're
    // closed. Here, we need to return a Promise that resolves to an
    // iterator result object.
    next() {
        return this.dequeue().then(value => (value === AsyncQueue.EOS)
                                   ? { value: undefined, done: true }
                                   : { value: value, done: false });
    }
}

// A sentinel value returned by dequeue() to mark "end of stream" when closed
AsyncQueue.EOS = Symbol("end-of-stream");

// Push events of the specified type on the specified document element
// onto an AsyncQueue object, and return the queue for use as an event stream
function eventStream(elt, type) {
    const q = new AsyncQueue();                  // Create a queue
    elt.addEventListener(type, e=>q.enqueue(e)); // Enqueue events
    return q;
}

async function handleKeys() {
    // Get a stream of keypress events and loop once for each one
    for await (const event of eventStream(document, "keypress")) {
        console.log(event.key);
    }
}