var Dog = function() {

    var _instance = 'hello world';

    return function() {
        console.log(this._instance);
    }
} (); //note that it is self invoking function

var l = new Dog(); //#> undefined 

In the above case I was expecting output of 'hello world'.

Why this._instance is not accessing the the varible which should be accessible by virtue of closure. I tested this in FF and am getting undefined.