The scope changes when you enter that anonymous closure and 'this' changes. You can hack around it by doing
var self = this;
And then using self in place of this (eg):
function Goal(name) {
var self = this;
/* ... */
this.gDiv.addEventListener('click', function(){
alert(self.tasks);
});
If you're using jQuery you could do something nicer:
this.gDiv.addEventListener('click', $.proxy(function() {
alert(this.tasks);
}, this));
Either way works just fine.
EDIT: In ES6, arrow functions can be used instead as they don't bind their own "this", so it becomes even simpler:
this.gDiv.addEventListener('click', () => {
alert(this.tasks);
});