executor.js

'use strict';

class Executor {
  constructor(maxTasksInParallel) {
    this._maxTasksInParallel = maxTasksInParallel;
    this._currentTasks = 0;
    this._queue = [];
  }

  start(task) {
    if (this._currentTasks < this._maxTasksInParallel) {
      this._currentTasks++;
      return task().then(this._taskFinished.bind(this), this._taskFinished.bind(this));
    } else {
      return new Promise((resolve, reject) => {
        this._queue.push([task, resolve, reject]);
      });
    }
  }

  _taskFinished(result) {
    if (this._queue.length) {
      var next = this._queue.pop();
      var task = next[0]
      var resolve = next[1];
      var reject = next[2];
      task().then(this._taskFinished.bind(this), this._taskFinished.bind(this)).then(resolve, reject);
    } else {
      this._currentTasks--;
    }
    return result;
  }
}
var e = new Executor(5);
for(let i = 0; i < 10; ++i) {
  e.start(function() {
    return new Promise(function(resolve, reject) {
      setTimeout(function() {
        console.log(i);
        resolve(i);
      }, i*100);
    })
  }).then(function(r) {
    console.log(r);
  });
}