mapLimit

function mapLimit(arr, limit, iterator) {
    let count = limit;
    const next = () => {
        if (count > arr.length) {
            return;
        }
        iterator(arr[count], next);
        count++;
    }
    for (let i = 0; i < limit; i++) {
        iterator(arr[i], next);
    }
}


// example
const data = [1, 2, 3, 4, 5];
const say = function(msg) {
    return new Promise(resolve => {
        setTimeout(() => {
            console.log(msg);
            resolve(msg);
        }, Math.random() * 1000 * 5);
    });
}
mapLimit(data, 2, (item, next) => {
    say(item).then(next);
});