http://www.youtube.com/watch?feature=player_detailpage&v=ya4UHuXNygM#t=2598 http://jsbin.com/ajoFEF/2/edit

promise.js

function make_promise() {
  var status = 'unresolved',
      outcome,
      waiting = [],
      dreading = [];
  
  function vouch (deed, func) {
    switch (status) {
      case 'unresolved':
        (deed === 'fulfilled' ? waiting: dreading).push(func);
        break;
      case deed:
        func(outcome);
        break;
    }
  }
  
  function resolve(deed, value) {
    if (status !== 'unresolved') {
      throw new Error('The promise has already been resolved: ' + status);
    }
    status = deed;
    outcome = value;
    (deed === 'fulfilled' ? waiting: dreading).forEach(function (func) {
      try {
        func (outcome);
      } catch (ignore) {}
    });
    waiting = null;
    dreading = null;
  }
  
  return {
    when: function (func) {
      vouch('fulfilled', func);
    },
    fail: function (func) {
      vouch('smashed', func);
    },
    fulfill: function (value) {
      resolve('fulfilled', value);
    },
    smash: function (string) {
      resolve('smashed', string);
    },
    status: function () {
      return status;  
    }
  };
}