javascript callback scope

i'm carrying problem plain aged javascript (no frameworks) referencing vigilant callback function.

function foo(id) {
    this.dom = document.getelementbyid(id);
    this.bar = 5;
    var self = this;
    this.dom.addeventlistener("click", self.onclick, false);
}

foo.prototype = {
    onclick : function() {
        this.bar = 7;
    }
};

now i emanate new vigilant (after dom loaded, span#test)

var x = new foo('test');

the 'this' inside onclick duty points span#test foo object.

how i anxiety foo vigilant inside onclick function?