gistfile1.js

var createObservedObject = function(callback) {
  var obj = {};
  var args = [].slice.call(arguments, 1);
  for(var i in args) {
    var thisPropertyName = args[i];
    (function(thisPropertyName) {
      obj['_' + thisPropertyName] = undefined;

      Object.defineProperty(obj, thisPropertyName, {
        get: function() {
          return obj['_' + thisPropertyName];
        },
        set: function(val) {
          callback('update', thisPropertyName, val);
          obj['_' + thisPropertyName] = val;
        }
      });
    })(thisPropertyName);
    
  }

  obj.addObservedProperty = function(propertyName) {
    obj['_' + propertyName] = undefined;

    Object.defineProperty(obj, propertyName, {
      get: function() {
        return obj['_' + propertyName];
      },
      set: function(val) {
        callback('add', propertyName, val);
        obj['_' + propertyName] = val;
      }
    });
  };

  return obj;
};

a = createObservedObject(console.log, 'arg1', 'arg2', 'arg3');
a.arg1 = 12;
console.log(a.arg1);
a.arg2 = 14;
console.log(a.arg2);
a.addObservedProperty('arg4');
a.arg4 = 16;
console.log(a.arg4);
a.unObserved = 'hi mom';
console.log(a.unObserved);