why projection is undefined using google maps v3 .

Question

this is my code:

    var i=0
    google.maps.event.addListener(map,'bounds_changed', function (){
        var bounds_=map.getBounds();
        if(!i){
            set_node_to_top()
        i=1;
    });
    function set_node_to_top(){
        MyOverlay.prototype = new google.maps.OverlayView();
        MyOverlay.prototype.onAdd = function() { }
        MyOverlay.prototype.onRemove = function() { }
        MyOverlay.prototype.draw = function() { }
        function MyOverlay(map) { this.setMap(map); }
        var overlay = new MyOverlay(map);
        var projection = overlay.getProjection();
    }

but ,the projection is undefined,

how can i do this ?

thanks