< select id = "sel" >
< option value = "1" >1</ option >
< option value = "2" selected = "selected" >2</ option >
< option value = "3" >3</ option >
</ select >

< script type = "text/javascript" >
function change(){
  document.getElementById("sel")[2].selected=true;
}
</ script >
< select id = "sel" >
< option value = "1" >1</ option >
< option value = "2" >2</ option >
< option value = "3" >3</ option >
</ select >
< input type = "button" value = "修改" onclick = "change()" />

var val = document.all.Item.options[document.all.Item.selectedIndex].text
var i=document.getElementById( 'sel' ).options[document.getElementById( 'sel' ).selectedIndex].value;

function createSelect(){
var mySelect = document.createElement( "select" );
mySelect.id = "mySelect" ;
document.body.appendChild(mySelect);
}

function addOption(){
//根據(jù)id查找對(duì)象，
var obj=document.getElementById( 'mySelect' );
//添加一個(gè)選項(xiàng)
obj.add( new Option( "文本" , "值" ));
}

function removeAll(){
var obj=document.getElementById( 'mySelect' );
obj.options.length=0;
}

function removeOne(){
var obj=document.getElementById( 'mySelect' );
//index,要?jiǎng)h除選項(xiàng)的序號(hào)，這里取當(dāng)前選中選項(xiàng)的序號(hào)
var index=obj.selectedIndex;
obj.options.remove(index);
}

var obj=document.getElementById( 'mySelect' );
var index=obj.selectedIndex; //序號(hào)，取當(dāng)前選中選項(xiàng)的序號(hào)
var val = obj.options[index].value;

var obj=document.getElementById( 'mySelect' );
var index=obj.selectedIndex; //序號(hào)，取當(dāng)前選中選項(xiàng)的序號(hào)
var val = obj.options[index].text;

var obj=document.getElementById( 'mySelect' );
var index=obj.selectedIndex; //序號(hào)，取當(dāng)前選中選項(xiàng)的序號(hào)
var val = obj.options[index]= new Option( "新文本" , "新值" );

function removeSelect(){
var mySelect = document.getElementById( "mySelect" );
mySelect.parentNode.removeChild(mySelect);
}