parallel tasks in pure JS - sample running here http://jsbin.com/weyiy/latest

parallelCbs.js

// process items in parallel thingy
var parallelCbs = function(items, itemFn, cb) {
  var left = items.length;
  var res = new Array(left);
  
  var cb2 = function(err, res) { // to enforce the main cb is not called more than once
    if (this.called) { return; }
    this.called = true;
    this.cb(err, res);
  }.bind({cb:cb, called:false});
  cb = cb2;
  
  var innerCb = function(err, innerRes) { // keeps track of remaining tasks
    if (err) { return cb(err); }
    --left;
    res[this.index] = innerRes;
    if (left !== 0) { return; }
    cb(null, res);
  };
  
  items.forEach(function(item, index) { // invoke the function
    try {
      itemFn(item, innerCb.bind({index:index}));
    } catch (ex) {
      return cb(ex);
    }
  });
};