How can I reference a function inside of a function using OOP.

I am trying to do this.

Code JavaScript:
with_field.onkeydown = function(event){
	associate_search.fill_list.down_key(event);
}

Where in another file I have this type of setup.

Code JavaScript:
function associate_search (){
     this.fill_list      =  fill_list;
 
     function fill_list(){
           //do stuff
 
           function down_key(event){
                  //do stuff
           }
     }
}
 
var associate_search	= new associate_search;

How can I reference the down_key function?