Q570 How can I dynamically add a new option to a select list?

var arrThemeSubs = new Array()
var arrThemeSubsID = new Array()

arrThemeSubsID[0] = "1"
arrThemeSubsID[1] = "2"
arrThemeSubsID[2] = "3"

arrThemeSubsName[0] = "sub1"
arrThemeSubsName[1] = "sub2"
arrThemeSubsName[2] = "sub3"

function addThemeSubs() {
  for (var i=0; i < arrThemeSubs.length ;i++) {	
    var newOpt  = new Option(arrThemeSubsName[i],arrThemeSubsID[i]);
    var themesubLength = document.form.themesub.length;
    document.form.themesub.options[themesubLength] = newOpt;
  }
}