new Option()——实现时间联动

1、基础准备：

　　先来了解下，如何运用js实现select动态添加option。

//1.动态创建select
function createSelect(){
    var mySelect = document.createElement("select");
    mySelect.id = "mySelect"; 
    document.body.appendChild(mySelect);
}

//2.添加选项option
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：
    }
}

//3.删除所有选项option
function removeAll(){
    var obj=document.getElementById('mySelect');
    obj.options.length=0;
}

//4.删除一个选项option
function removeOne(){
    var obj=document.getElementById('mySelect');
    var index=obj.selectedIndex;    //index,要删除选项的序号
    obj.options.remove(index);
}

//5.获得选项option的值
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;

//6.获得选项option的文本
var obj=document.getElementById('mySelect');
var index=obj.selectedIndex;    
var val = obj.options[index].text;

//7.修改选项option
var obj=document.getElementById('mySelect');
var index=obj.selectedIndex;    
var val = obj.options[index]=new Option("新文本","新值");

//8.删除select
function removeSelect(){
    var mySelect = document.getElementById("mySelect");
    mySelect.parentNode.removeChild(mySelect);
}

2、时间联动实战

　　demo:

　　时间联动的要点就是天数随月份变化，考虑闰年，口诀：四年一润，百年不润，四百年再润。下面直接看代码吧。

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
    <select id="year" name="year">
         <option>年份</option>
    </select>
    <select id="month" name="month">
         <option>月份</option>
    </select>
    <select id="day" name="day">
         <option>日</option>
    </select>
    <script>
        function formatDate(){};    
        formatDate.prototype={
            init:function(){
                var _this = this;
                yObj = document.getElementById("year");
                mObj = document.getElementById("month");
                dObj = document.getElementById("day");
                this.yObj = yObj;
                this.mObj = mObj;
                this.dObj = dObj;
                var year = new Date().getFullYear();

                for(var i = year;i >= 1900;i--){
                    yObj.options.add(new Option(i,i));
                }

                for(var j=1;j<=12;j++){
                    mObj.options.add(new Option(j,j));
                }    

                yObj.onchange = function(){
                    _this.day();
                }
                mObj.onchange = function(){
                    _this.day();
                }
            },
            day:function(){
                this.dObj.options.length = 1
                var y = this.yObj.options[yObj.selectedIndex].value;
                var m = this.mObj.options[mObj.selectedIndex].value;
                var days = [31,28,31,30,31,30,31,30,30,31,30,31]
                if((y%4 == 0 && y%100 != 0) || y%400 == 0 ){
                    days[1]++;
                }
                for(var k = 1;k <= days[m-1]; k++){
                    this.dObj.options.add(new Option(k,k));
                }
        }
    }


    var f = new formatDate();    
    f.init();
    </script>

</body>
</html>