JS dynamically adds and deletes Option objects in select (example code)

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

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

   var selectObj=("user_dm"); 
   alert(); 
   [] = new Option("mytest", "2"); 
}