gistfile1.js

function ObservableProperty(obj, key) {
  this.obj = obj;
  this.key = key;
}

ObservableProperty.prototype.get = function() {
  return this.obj.get(this.key);
};

ObservableProperty.prototype.addObserver = function(target, method) {
  this.obj.addObserver(this.key, target, method);
};

ObservableProperty.prototype.removeObserver = function(target, method) {
  this.obj.removeObserver(this.key, target, method);
};

function getProperty(obj, key) {
  return new ObservableProperty(obj, key);
}