Another minimal, but more feature rich Promise implementation

gistfile1.js

var Promise = (function() {
  var api_tuples = {notify: 'progress', resolve: 'then', reject: 'fail', done: 'always' }, // all (action, callback_adder)-tuples)
    isFunction = function(obj) {
      return Object.prototype.toString.call(obj) == '[object Function]';
    },
    slice = Array.prototype.slice,
    each_pair = function(obj, callback) {
      Object.keys(obj).forEach(function(key) { callback(key, obj[key]); });
    },
    invokeEach = function(funcs) {
      for (var i = 0, len = funcs.length; i < len; i++) {
        funcs[i].apply(null, slice.call(arguments, 1));
      }
    },
    return_false = function() { return false; },
    return_true = function() { return true; };
  return function() {
      var self = {isResolved: return_false, isRejected: return_false},
      callbacks = {},
      fullfill = function(type, value) {
        self.resolve = self.reject = function() { throw new Error('Promise already completed'); };
        self[api_tuples[type]] = function(callback) { callback(value); }; // Switch to synchronized
        invokeEach(callbacks[type], value);
        self[type === 'resolve' ? 'isResolved' : 'isRejected'] = return_true;
        invokeEach(callbacks.done, value);
        callbacks = null;
      };
    each_pair(api_tuples, function(action, callback_adder) {
      var _callbacks = callbacks[action] = [];
      self[callback_adder] = function(callback) {
        if (!isFunction(callback)) { throw new Error("Callback for "+callback_adder+" must be a function."); }
        _callbacks.push(callback);
      };
      if (action === 'done') { return; } // done is internal, do not expose
      self[action] = (action === 'notify') ?
          function(value) { invokeEach(_callbacks, value); } : function(value) { fullfill(action, value); };
    });
    return self;
  };
}());