quick promise based queue sketch

promise-queue.js

function queue(concurrency) {
  var outstanding = [];
  return function(fn) {
    outstanding.push(fn);
    if(outstanding.length < concurrency) {
      pop();
    }
  }

  function pop() {
    if(outstanding.length > 0) {
      var getPromise = outstanding[0];
      getPromise().finally(function() {
        outstanding.shift();
        pop();
      });
    }
  }
}