javascript - Scoping Issue inside a function

I am having an issue with the scope of a variable. The first two return the correct value but when I subscribe to an event (the event is working) it returns this._id as undefined. I have also tried it as MyFunction._id;

 var i = 0;


 var func = new MyFunction();
 func.init();

 function MyFunction(i){
     this._id = i;
 }

 MyFunction.prototype.init = function(){

     Debugger.log("A : " + this._id); //displays the result of i

     this.myTest; //displays the result of i
     Event.subscribe("UPDATE", this.myTest);// is undefined
 }

 MyFunction.prototype.myTest = function(){
     Debugger.log("B : " + this._id);
 }

Thx.

Answer

You have to do:

 Event.subscribe("UPDATE", this.myTest.bind(this));