如何判断select中是否包含一个值

[haochangjun]

代码:

<input type="text" id="AreaName" name="AreaName" class="input_blue" maxlength="63" size="14">

<select name="selectItem" style="z-index: 102; left: 264px; width: 165px;" id="selectItem" multiple="" size="5"><option value="DDDDD">DDDD</option></select>

<input type="submit" src="../../../img/button_next.gif" value="增加">

当我点“增加 ”的时候就将ID为AreaName中的值 插入到select中的option中去，这个已经实现了，现在的问题是假如一条记录已经添加了，再添加的时候如何不让添加进去，也就是判断select，option中已经包含了AreaName中的值了

[toury]

var o=document.getElementById('selectItem');
var s="test"
for(var i=0;i<o.options.length;i++){
if(o.options[i].value==s){ alert('exsisted!');return }
}

[lemon520]

function addToSelect(value) {
  var opts = document.getElementById('selectItem').options;
  for(var i=0, n=opts.length; i<n; i++){
    if(opts[i].value == value) return;
  }
  var opt = document.createElement("OPTION");
  opt.value = value;
  opt.innerText = value;
  opts.add(opt);
}

[zyzy15]

<input type="text" id="AreaName" name="AreaName" class="input_blue" maxlength="63" size="14">

<select name="selectItem" style="z-index: 102; left: 264px; width: 165px;" id="selectItem" multiple="" size="5"><option value="DDDDD">DDDD</option></select>

<input type="button" src="../../../img/button_next.gif" value="增加" onclick="fun()">
<SCRIPT LANGUAGE="JavaScript">
<!--
	function fun(){
		var AreaName = document.getElementById("AreaName");
		var selectItem = document.getElementById("selectItem");
		if (AreaName.value)
		{
			for (var i=0; i<selectItem.options.length; i++)
			{
				if (selectItem.options[i].value == AreaName.value) return;
			}
			var opt = document.createElement("option");
			opt.value = opt.innerHTML = AreaName.value;
			selectItem.appendChild(opt);
		}
	}
//-->
</SCRIPT>