Promise_Pool.js

/**
 * @param {Function[]} functions
 * @param {number} n
 * @return {Function}
 */
var promisePool = async function(functions, n) {
    return new Promise((resolve,reject)=>{
        let i=0;
        let inprogress=0;
        function callback() {
            if (i===functions.length && inprogress===0)
            {
                resolve();
            }
            while (i<functions.length && inprogress<n)
            {
                functions[i++]()
                    .then(()=>
                    {
                        inprogress -- ;
                        callback();
                    });
                inprogress ++;
            }
        }
        callback();
    });
};

/**
 * const sleep = (t) => new Promise(res => setTimeout(res, t));
 * promisePool([() => sleep(500), () => sleep(400)], 1)
 *   .then(console.log) // After 900ms
 */