Mapping an array asynchronously with maximum N number of concurrent requests

asyncMap.js

const parallelMap = (arr, fn, max) => {
    const out = [];
    const initial = [...Array(max)];
    const fired = initial.map(() => Promise.resolve().then(async function cb() {
        if (!arr.length) return
        
        const next = arr.shift();
        out.push(await fn(next));
        return cb();
    }));
    return Promise.all(fired).then(() => out);
};