html中关于<select>标签如何设置默认选中的选项详解

< 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()"  />

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);
}