//城市
    var city = ['请选择','北京','天津','上海','重庆'];
    //地区
    var district = [[],
        ['东城','西城','朝阳','海淀'],
        ['河东','河西','河北','南开'],
        ['黄埔','徐汇','长宁','静安'],
        ['渝北','渝中','江北','江津']];
    window.onload=function(){
        createCity();
        document.getElementById("city").onchange= createDistrict;
    };
    function createCity(){
        //获得1级菜单select
        var ci = document.getElementById("city");
        //为1级菜单select添加option
        for(var i in city){
            var op = new Option(city[i],city[i]);
            ci.options.add(op);
        }
    }
    function createDistrict(){
        //获取当前选中的一级菜单的选项的下标
        var index = document.getElementById("city").selectedIndex;
        //获得2级菜单select
        var di = document.getElementById("district");
        //清空二级菜单选项
        di.options.length=0;
        //为2级菜单select添加option
        for(var i in district[index]){
            var op = new Option(district[index][i],district[index][i]);
            di.options.add(op);
        }
    }
</script>

body部分：

<p>请选择所述地区：</p>
<select id="city"></select>
<select id="district"></select>

分析：此次的js二级联动只是充分运用数组，js中的selectedIndex，以及为*级菜单select添加option,简单实用。