Promise All with a max number of concurrent tasks

promiseAllWithLimit.js

/**
 * Run promise with a max number of concurrent tasks
 * @param {Number} limit 
 */
function promiseAllWithLimit(limit) {
    let count = 0;
    let queue = [];

    const run = (fn, resolve, reject, ...args) => {
        count++;
        const promise = fn.apply(this, args);

        return promise
            .then((res) => {
                count--;

                if(queue.length > 0) {
                    queue.shift()();
                }
                
                resolve(res);
            })
            .catch(err => {
                count--;
                reject(err);
            });
    }

    const helper = (fn, ...args) => {

        return new Promise((resolve, reject) => {
            if(count < limit) {
                run(fn, resolve, reject, ...args);
            } else {
                queue.push(run.bind(this, fn, resolve, reject, ...args))
            }
        });
    }

    return helper;
}

// --------TEST--------
// each time only run 3 promise
// once there is a promise done, the next promise will be run immediately
var pl = pLimit(3); 

const p = (duration) => {
    return new Promise((resolve, reject) => {
        console.log('Running', duration);
        setTimeout(() => {
            if(duration > 4) {
                reject(duration);
                return;
            }
            resolve(duration);
        }, duration * 1000);
    })
}

Promise.all([
    pl(() => p(10)),
    pl(() => p(5)),
    pl(() => p(1)), // run first 3
    pl(() => p(9)), // after that, run each by each
    pl(() => p(6)),
    pl(() => p(4)),
    pl(() => p(1)),
    pl(() => p(2)),
    pl(() => p(10)),
])
.then(res => {
    console.log('Promise all!!!!', res);  // 'Promise all!!!!', [10, 5, 1, 9, 6, 4, 1, 2, 10] 
})
.catch(err => {
    console.error(err);
})