var myObj = SC.Object.create({


  // so either like this
  _cachedVal: "",

  value: function(key,value){
    if(value){
      this._cachedVal = value;
    }
    return this._cachedVal;
  }.property(),

  // or like this:

  value2: function(key,value){
    if(value) return value;
    else return "";
  }.property().cacheable()


})