<SELECT NAME="aaa" SIZE="" style="width:200px" >

<OPTION VALUE="">( empty )</OPTION>
<OPTION VALUE="1">1</OPTION>
</SELECT>

//Get the HTML control
var select = document.getElementsByName("aaa")[0];

select.selectedIndex = index;
//Create a new Option object
new Option(text,value)
new option(text,value,defaultSelected,selected
text: String, specify option The object's text attribute ( namely <option></option> Between words )
value: String, specify option The object's value attribute
defaultSelected: Boolean value, specified option The object's defaultSelected attribute
selected: Boolean value, specified option The object's selected attribute

//Add options to select
select.add(new Option(text,value))

// delete
select.options.remove(index)
//Delete all at once
select.length = 0;