select标签设置默认选中的选项方法

< 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(){
//根据id查找对象，
var obj=document.getelementbyid( 'myselect' );
//添加一个选项
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,要删除选项的序号，这里取当前选中选项的序号
var index=obj.selectedindex;
obj.options.remove(index);
}

var obj=document.getelementbyid( 'myselect' );
var index=obj.selectedindex; //序号，取当前选中选项的序号
var val = obj.options[index].value;

var obj=document.getelementbyid( 'myselect' );
var index=obj.selectedindex; //序号，取当前选中选项的序号
var val = obj.options[index].text;

var obj=document.getelementbyid( 'myselect' );
var index=obj.selectedindex; //序号，取当前选中选项的序号
var val = obj.options[index]= new option( "新文本" , "新值" );

function removeselect(){
var myselect = document.getelementbyid( "myselect" );
myselect.parentnode.removechild(myselect);
}