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"));    //方法一：
    
    obj.options.add(new Option("text","value")); //方法二：
    
    for(var i=0;i<10;i++){
        obj.options[i]=new Option("新文本","新值");    //方法三：
    }
}

function removeAll(){
    var obj=document.getElementById('mySelect');
    // 当index不为0时，为部分删除，只删除index后面的选项
    obj.options.length=0;
}

function removeOne(){
    var obj=document.getElementById('mySelect');
    var index=obj.selectedIndex;    //index,要删除选项的序号
    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;    
var val = obj.options[index].text;

var obj=document.getElementById('mySelect');
var index=obj.selectedIndex;    
var val = obj.options[index]=new Option("新文本","新值");

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