why.js

var MyClass = function() {
        this.cosa = 0;
        console.log('constructor');
};

(function() {
        var func1 = function() {
                console.log('func1');
                console.log(this.cosa);
        };

        this.func2 = function() {
                console.log('func2');
                func1.call(this);
        };
}).call(MyClass.prototype);

var i = new MyClass();
i.func2();