Override Combo's onCurrentItemChanged method

Hello,

Is there a way to override the onCurrentItemChanged method of a Combo so it does something else before showing the newly selected item on the control?

Something like:

cmb.onCurrentItemChanged = function(){ Validacion(this);}

var Validacion=function(e){
                alert('Whatever');
                original_onCurrentItemChanged_functionality();
            };  Thank you in advance.