Combo.js

// combo library
function Combo(callback) {
  this.callback = callback;
  this.items = 0;
  this.results = [];
}
Combo.prototype = {
  add: function () {
    var self = this;
    this.items++;
    return function () {
      self.check(self.items - 1, arguments);
    };
  },
  check: function (id, arguments) {
    this.results[id] = arguments;
    this.items--;
    if (this.items == 0) {
      this.callback.apply(this, this.results);
    }
  }
};

// Usage
var both = new Combo(function () {
  puts(inspect(arguments));
});
setTimeout(both.add(), 100);
setTimeout(both.add(), 50);