for await vs Promise.all

demo-for-await-vs-promise-all.js

a = [1,2,3,4,5,5,6,7];
op = e => new Promise(s=>setTimeout(()=>{ console.log("N:", e); s(e)}, Math.random()*1000))

do1 = async () => { await Promise.all(a.map(op)); }
do2 = async ()=>{ for await (const e of a) await op(e) }

await do1(); console.log('done');
/* OUTPUT Promise.all
N: 6
N: 5
N: 2
N: 5
N: 4
N: 3
N: 1
N: 7
done
*/

await do2(); console.log('done');

/* OUTPUT for await
N: 1
N: 2
N: 3
N: 4
N: 5
N: 6
N: 7
done
*/

// Hide the imperative "for await" for modern funcional prograaming
const forEachAwait = async (a, op) => { for await (const e of a) await op(e) };

// Try it with a generator
function* makeRangeIterator(start = 0, end = 100, step = 1) {
    let iterationCount = 0;
    for (let i = start; i < end; i += step) {
        iterationCount++;
		    console.log('G:', i);
        yield i;
    }
    return iterationCount;
}

// A Slower operation
const slowOp = e => new Promise(s=>setTimeout(()=>{ console.log("N:", e); s(e)}, Math.random()*5000))

// See how the generator goes one step at a time, and each item waits for the previous one.
forEachAwait(makeRangeIterator(1,10), slowOp);