How to add multiple callbacks to a javascript method?

I have created a simple javascript class using the 'function' technique. In the class I have a websocket listener which triggers a function when a certain message is received. I could easily add an external callback to it, as follows

function MyClass() {
    self = this; // to access main object from methods

    // websocket definition

    function websocketMessageInterpreter(message){
       if(message == "Hello!")  onHelloMessageBase();
     }

    function onHelloMessageBase(param){

    // call user defined callback
    self.userDefinedCallback(param);
    }
    this.userDefinedCallback= function(param){}; //default empty callback
}

Externally I use this as

var myObject = new MyClass();
myObject.userDefinedCallback = function(){alert("Someone said hello!");};

Is there a model where I could do something like this?

myObject.userDefinedCallback += function1;
myObject.userDefinedCallback += function2;

and maybe later

myObject.userDefinedCallback -= function1;