gistfile1.js

var assert = require('assert');
var s = '';
var append_char = function(arg) {
  s+= arg;
};

function rendezvous(callback) {
  return {
    callbacks: [],
    callback: function(cb) {
      var rdv = this;
      rdv.callbacks.push(cb);
      return function() {
	rdv.callbacks.splice(rdv.callbacks.indexOf(cb), 1)[0].apply(this, arguments);
	if (rdv.callbacks.length === 0) {
	  callback();
	}
      };
    }
  };
}

var rdv_up = rendezvous(function() {
  assert.equal('3210', s);
  console.log('OK');  
});

var rdv = rendezvous(rdv_up.callback(function() {
  append_char('0');
}));

var f1 = rdv.callback(append_char);
var f2 = rdv.callback(append_char);
var f3 = rdv.callback(append_char);

f3('3');
f2('2');
f1('1');