javascript 操作select下拉列表框的一点小经验

按照我一贯的web开发风格，所有不直接操作数据库的事件，都尽可能由javascript来实现，所以这个需求我打算使用js来完成。
首先来分析一下具体情况：这个页面是一个更新页面，品牌有品牌1和品牌2两个字段，品牌2可以为空，品牌1不能为空，所以品牌2的下拉列表框比品牌1多一项；如果选择了品牌的前8相中的任意一项，“活跃状态”要隐藏，否则“活跃状态”默认显示状态为“潜在”；当查询的结果品牌1和品牌2有任意一项在品牌的前8相中，“活跃状态”也要隐藏，否则“活跃状态”默认显示状态为“潜在”。
页面部分内容

<div id="div_Brand_Baby" name="div_Brand_Baby" style="display: none" runat="server"> 
<div style="float: left;">品牌1：</div> 
<div style="position: relative; float: left;"> 
<span style="margin-left: 170px; width: 18px; overflow: hidden;"> 
<atlas:UpdatePanel ID="UpdatePanel12" runat="server"> 
<ContentTemplate> 
<asp:DropDownList ID="ddlistFirstConsumeBrand" onchange="ChangeBrand(this)" runat="server" 
DataTextField="OptionText" DataValueField="optionValue" DataSourceID="ObjectDataSource11" 
Style="width: 188px; margin-left: -170px"> 
</asp:DropDownList> 
</ContentTemplate> 
</atlas:UpdatePanel> 
</span> 
<asp:TextBox ID="txtBrand1" runat="server" onblur="changebrand1(this)" Style="width: 170px; 
position: absolute; left: 0px;"></asp:TextBox> 
</div> 
<div style="float: left;"> 
  品牌2：</div> 
<div style="position: relative; float: left;"> 
<span style="margin-left: 170px; width: 18px; overflow: hidden;"> 
<atlas:UpdatePanel ID="UpdatePanel13" runat="server"> 
<ContentTemplate> 
<asp:DropDownList ID="ddlistSecondConsumeBrand" runat="server" onchange="ChangeBrand(this)" 
DataTextField="OptionText" DataValueField="optionValue" DataSourceID="ObjectDataSource12" 
Style="width: 188px; margin-left: -170px"> 
</asp:DropDownList> 
</ContentTemplate> 
</atlas:UpdatePanel> 
</span> 
<asp:TextBox ID="txtbrand2" runat="server" onblur="changebrand1(this)" Style="width: 170px; 
position: absolute; left: 0px;"></asp:TextBox> 
</div> 
<asp:ObjectDataSource ID="ObjectDataSource11" runat="server" SelectMethod="RetrieveMilkBrand_Baby" 
TypeName="CRR.BusinessRules.OptionManager"> 
<SelectParameters> 
<asp:Parameter DefaultValue="1" Name="languageID" Type="Int32" /> 
<asp:Parameter DefaultValue="false" Name="addNull" Type="Boolean" /> 
</SelectParameters> 
</asp:ObjectDataSource> 
<asp:ObjectDataSource ID="ObjectDataSource12" runat="server" SelectMethod="RetrieveMilkBrand_Baby" 
TypeName="CRR.BusinessRules.OptionManager"> 
<SelectParameters> 
<asp:Parameter DefaultValue="1" Name="languageID" Type="Int32" /> 
<asp:Parameter DefaultValue="true" Name="addNull" Type="Boolean" /> 
<asp:Parameter DefaultValue=" " Name="nullString" Type="String" /> 
</SelectParameters> 
</asp:ObjectDataSource> 
</div>

javascript代码

function changebrand1(oTextbox) 
{ 
var brandTag=document.getElementById("ddlistSecondConsumeBrand"); 
var brand1=document.getElementById("txtbrand1"); 
var brand2=document.getElementById("txtbrand2"); 
var brandcolls=brandTag.options; 
var textvalue=oTextbox.value; 
var flag=0; 
if(textvalue.length==0) 
{ 
flag=1; 
} 
else if(textvalue.length>0) 
{ 
for(var i=0;i<brandcolls.length;i++) 
{ 
if(oTextbox==brand1 && brandcolls[i].text==textvalue) 
{ 
document.getElementById("ddlistFirstConsumeBrand").options.selectedIndex=i-1; 
flag=1; 
ChangeBrand(document.getElementById("ddlistFirstConsumeBrand")); 
} 
else if(oTextbox==brand2 && brandcolls[i].text==textvalue) 
{ 
brandTag.selectedIndex=i; 
flag=1; 
ChangeBrand(brandTag); 
} 
} if(flag==0) 
{ 
alert("输入品牌错误！"); 
oTextbox.value=""; 
oTextbox.focus(); 
} 
} 
} 
function ChangeBrand(me){ 
var brand1ID = document.all.ddlistFirstConsumeBrand.value; 
var brand2ID = document.all.ddlistSecondConsumeBrand.value; 
var brandvalue1=document.getElementById("txtbrand1"); 
var brandvalue2=document.getElementById("txtbrand2"); 
if((brand1ID=="10")&&(brand2ID=="-1")) 
{ 
document.all.ddlistMilkPeriod.value=9; 
} 
for(var i=0;i<document.getElementById("ddlistSecondConsumeBrand").options.length;i++) 
{ 
if(document.getElementById("ddlistFirstConsumeBrand") == me && document.all.ddlistFirstConsumeBrand.selectedIndex==i) 
{ 
brandvalue1.value=document.getElementById("ddlistFirstConsumeBrand").options[i].text; 
} 
if(document.getElementById("ddlistSecondConsumeBrand") == me && document.all.ddlistSecondConsumeBrand.selectedIndex==i) 
{ 
brandvalue2.value=document.getElementById("ddlistSecondConsumeBrand").options[i].text; 
} 
if(i<8 && document.getElementById("ddlistFirstConsumeBrand") == me && document.all.ddlistFirstConsumeBrand.selectedIndex==i) 
{ 
document.all.dv1.style.display="block"; 
document.all.dv2.style.display="none"; 
document.all.dv3.style.display="none"; 
document.getElementById("ddlistPotential").options[0].selected="selected"; 
break; 
} 
else if(i>0 && i<9 && document.getElementById("ddlistSecondConsumeBrand") == me && document.all.ddlistSecondConsumeBrand.selectedIndex==i) 
{ 
document.all.dv1.style.display="block"; 
document.all.dv2.style.display="none"; 
document.all.dv3.style.display="none"; 
document.getElementById("ddlistPotential").options[0].selected="selected"; 
break; 
} 
else if(i>8) 
{ 
document.all.dv1.style.display="none"; 
document.all.dv2.style.display="block"; 
document.all.dv3.style.display="block"; 
document.getElementById("ddlistPotential").options[1].selected="selected"; 
} 
} 
}