Homework - Method problem

I have a code which has an exception plz help me to find it out
I think the problem is for o.addto(12); but if yes I dont know how to fix it

thanks ..............

function addto(value){
myvar +=value ;
return ;
}

var o = new Object();
o.myvar=0;
o.addto=addto;
o.addto(12);

I solved it
function addto(value){
this.myvar +=value ;
return ;
}

but I dont know the reason :?