how to add to alert function the chosen list from dropdown

still very beginner in js…
how to add to alert function the chosen list from dropdown as the sample below ?
thanks!

e.g

enter image description here
enter image description here

var w = new Window ("dialog");
var col = w.add ("edittext", undefined, com[0]);
var list = ['A','-','B','-','C','-','D','-','E'];
var dropdown = w.add ("dropdownlist", undefined, list);  dropdown.selection = dropdown.items[0];
w.show ();

function com(){
   save = "column " + col.text + "+ list"; 
   return save
   }
alert(com());