I need some simple objects that could become more complex later, with many different properties, so i thought to decorator pattern. I made this looking at Crockford's power constructor and object augmentation:

//add property to object
Object.prototype.addProperty = function(name, func){
    for(propertyName in this){
        if(propertyName == name){
            throw new Error(propertyName + " is already defined");
        }
    }
    this[name] = func;
};

//constructor of base object
var BaseConstructor = function(param){

    var _privateVar = param;
    return{
        getPrivateVar: function(){
            return _privateVar;
        }
    };
};

//a simple decorator, adds one private attribute and one privileged method 
var simpleDecorator = function(obj, param){

    var _privateVar = param;

    var privilegedMethod1 = function(){
        return "privateVar of decorator is: " + _privateVar;
    };

    obj.addProperty("privilegedMethod1", privilegedMethod1);

    return obj;
}

//a more complex decorator, adds public and private properties
var complexDecorator = function(obj, param1, param2){

    //private properties
    var _privateVar = param1;
    var _privateMethod = function(x){
        for(var i=0; i<x; i++){
            _privateVar += x;
        }
        return _privateVar;
    };

    //public properties
    var publicVar = "I'm public";
    obj.addProperty("publicVar", publicVar);

    var privilegedMethod2 = function(){
        return _privateMethod(param2); 
    };
    obj.addProperty("privilegedMethod2", privilegedMethod2);

    var publicMethod = function(){
        var temp = this.privilegedMethod2();
        return "do something: " + temp + " - publicVar is: " + this.publicVar;
    };
    obj.addProperty("publicMethod", publicMethod);

    return obj;
}

//new basic object
var myObj = BaseConstructor("obj1");

//the basic object will be decorated
var myObj = simpleDecorator(obj, "aParam");

//the basic object will be decorated with other properties
var myObj = complexDecorator(obj, 2, 3);

Is this a good way to have Decorator Pattern in javascript? Are there other better ways to do this?