Async generators and "for await" in JavaScript

async-generator.js

// by @noseratio
// https://twitter.com/noseratio/status/1297517388552757249?s=20
// gist: https://gist.github.com/noseratio/721fea7443b74a929ea93c8f6a18cec4/edit
// RunKit: https://runkit.com/noseratio/async-generators-and-for-await-in-javascript

async function delay(ms) {
    await new Promise(r => setTimeout(r, ms));
    return ms;
}

// an array of promises to iterate through with "for await"
function array() {
    return [delay(100), delay(200), delay(300)];
}

// a synchronous generator of promises to iterate through with "for await"
function* generator() {
    yield delay(100);
    yield delay(200);
    yield delay(300);
}

// an asynchronous generator of promises to iterate through with "for await"
async function* asyncGenerator() {
    yield await delay(100);
    yield await delay(200);
    yield await delay(300);
}

// Simulate a "for await" loop in JavaScript,
// note that "await 1" is still a valid statement,
// which is essentially the same as "await Promise.resolve(1)"
async function forEachAsync(iterable, bodyFunc) {
    const iterator = (
        iterable[Symbol.asyncIterator] || 
        iterable[Symbol.iterator]).call(iterable);

    while (true) {
        const { value, done } = await iterator.next();
        const awaitedValue = await value;
        if (done) {
            return awaitedValue;
        }
        await bodyFunc(awaitedValue);
    }
}

// test with "for await"
async function testForAwait(iterable) {
    let start = Date.now();
    for await (let value of iterable) {
        console.log(`value: ${value}, lapse: ${Date.now() - start}`)
    } 
}

// test with "forEachAsync"
async function testForEach(iterable) {
    let start = Date.now();
    await forEachAsync(iterable, value => {
        console.log(`value: ${value}, lapse: ${Date.now() - start}`)
    })
}

// test all three sources
async function test(method) {
    await delay(100); // kick the runkit runtime

    console.log("- array")
    await method(array());

    console.log("- generator")
    await method(generator());

    console.log("- async generator")
    await method(asyncGenerator());
}

async function main() {
    console.log("testForAwait")
    await test(testForAwait);

    console.log("testForEach")
    await test(testForEach);
}

main().then(() => console.log("Finished"), e => console.log(e));

Inspired by this tweet of @bengl.

Thanks for sharing. How does the loop on lines L38-L45 exit, given while (true)?

@hvent90 the return statement L42 exits the function after going through every await message available