JS dynamically adds and deletes Option objects in select (sample code)_javascript skills

//Dynamicly delete all items in the select options:
function delAllOptions(){
document.getElementById("user_dm").options.length=0;
}

//Dynamicly delete an option in the select:
function delOneOption(index){
document.getElementById("user_dm").options.remove(index);
}
// Dynamically add item option in select:
function addOneOption (){
//document.getElementById("user_dm").options.add(new Option(2,"mytest"));

var selectObj=document.getElementById("user_dm");
alert(selectObj.length);
selectObj.options[selectObj.length] = new Option("mytest", "2");
}