How to use JavaScript to create a dynamic drop-down menu effect

nbsp;html>
    <meta>
    <title>利用二维数组创建动态下拉菜单</title>
    <script>
        //定义一个二维数组aCity,用于存储城市名称        
        var aCity=new Array();        
        aCity[0]=new Array();        
        aCity[1]=new Array();        
        aCity[2]=new Array();        
        aCity[3]=new Array();        
        aCity[4]=new Array();        
        aCity[5]=new Array();        //赋值,每个省份的城市存放在用于数组的一行        
        aCity[0][0]="--请选择--";        
        aCity[1][0]="--请选择--";        
        aCity[1][1]="东城区";       
        aCity[1][2]="西城区";        
        aCity[1][3]="朝阳区";        
        aCity[1][4]="海淀区";        
        aCity[1][5]="丰台区";        
        aCity[2][0]="--请选择--";        
        aCity[2][1]="福田区";        
        aCity[2][2]="罗湖区";        
        aCity[2][3]="南山区";        
        aCity[2][4]="宝安区";        
        aCity[2][5]="龙岗区";        
        aCity[2][6]="盐田区";        
        aCity[3][0]="--请选择--";       
         aCity[3][1]="越秀区";        
         aCity[3][2]="东山区";        
         aCity[3][3]="海珠区";        
         aCity[3][4]="荔湾区";        
         aCity[3][5]="天河区";        
         aCity[3][6]="白云区";        
         aCity[4][0]="--请选择--";        
         aCity[4][1]="西湖区";        
         aCity[4][2]="上城区";       
          aCity[4][3]="下城区";        
          aCity[4][4]="江干区";        
          aCity[4][5]="拱墅区";        
          aCity[4][6]="滨江区";        
          aCity[5][0]="--请选择--";        
          aCity[5][1]="新城区";        
          aCity[5][2]="碑林区";        
          aCity[5][3]="莲湖区";        
          aCity[5][4]="雁塔区";        
          aCity[5][5]="灞桥区";        
          aCity[5][6]="长安区";        
          aCity[5][7]="周至县";        
          function ChangeCity(){            
          var i,iProvinceIndex;            
          iProvinceIndex=document.frm.optProvince.selectedIndex;            
          iCityCount=0;            
          while(aCity[iProvinceIndex][iCityCount]!=null)                
          iCityCount++;                //计算选定省份的城市个数                
          document.frm.optCity.length=iCityCount; //改变下拉菜单的选项数                
          for(i = 0;i<=iCityCount-1;i++)  //改变下拉菜单的内容                    
          document.frm.optCity[i]=new Option(aCity[iProvinceIndex][i]);                    
          document.frm.optCity.focus();
        }    </script>
    <h3 id="选择省份及城市">选择省份及城市</h3>
    

        

省份:                     --请选择--             北京市             深圳市             广州市             杭州市             西安市                  

        

城市:                              --请选择--                  

    

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