A new flow control example, for cross reference

Trigger.js

var Trigger = function() {
  this.__callbacks = [];
  this.__done = [];
};

Trigger.prototype = {
  set: function(name) {
    var self = this;
    return function(err, result) {
      if (err)
        (self.errors || (self.errors = {}))[name] = err;
      else
        self[name] = result;

      self.__trigger(name);
    }
  },

  on: function() {
    var args = Array.prototype.slice.call(arguments);
    if (args.length < 2) {
      throw 'at least 2 arguments trigger.on("foo","bar",callback)';
    }
    this.__callbacks.unshift(args);
  },

  __trigger: function(name) {
    this.__done.push(name);
    for (var i = this.__callbacks.length - 1; i >= 0; i--) {
      var args = this.__callbacks[i],
          flag = true;
      for (var j = args.length - 2; j >= 0; j--) {
        flag = flag && (this.__done.indexOf(args[j]) >= 0);
      }
      if (flag) {
        args[args.length - 1]();
        this.__callbacks.splice(i, 1);
      }
    }
  }
};

// test code

function foo(name, time, callback) {
  setTimeout(function() {
      console.log('ready ' + name);
      callback(null, name);
  }, time);
}

var trigger = new Trigger();

foo('r0', 10, trigger.set('bar0'));
foo('r1', 20, trigger.set('bar1'));
foo('r2', 30, trigger.set('bar2'));
foo('r3', 40, trigger.set('bar3'));
trigger.on('bar2', 'bar1', function() {
    if (trigger.errors) {
    } else {
      console.log('on bar2,bar1,' + trigger.bar2 + trigger.bar1);
    }
});
trigger.on('bar2', 'bar3', function() {
    if (trigger.errors) {
    } else {
      console.log('on bar2,bar3,' + trigger.bar2 + trigger.bar3);
    }
});
trigger.on('bar0', 'bar1', function() {
    if (trigger.errors) {
    } else {
      console.log('on bar0,bar1,' + trigger.bar0 + trigger.bar1);
      foo(trigger.bar0 + trigger.bar1, 10, trigger.set('bar4'));
    }
});
trigger.on('bar0', 'bar2', function() {
    if (trigger.errors) {
    } else {
      console.log('on bar0,bar2,' + trigger.bar0 + trigger.bar2);
    }
});
trigger.on('bar2', 'bar4', function() {
    if (trigger.errors) {
    } else {
      console.log('on bar2,bar4,' + trigger.bar2 + trigger.bar4);
    }
});