select 或text的onchange事件需要手動（透過鍵盤輸入）改變select或text的值才能觸發，如果在js中給select或text賦值，則無法觸發onchang事件，
例如，在頁面載入完成以後，需要觸發一個onChange事件，在js中用document.getElementById("province").value="湖北";直接給select或text賦值是不行的，要實現手動觸發onchange事件，需要在js將select賦值後，加入下面的語句
document.getElementById("province").fireEvent('onchange') 來實作,
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>无标题文档</title>
<script type="text/javascript">
var provinces = new Array();
provinces["湖北"] = ["武汉","襄阳","随州","宜昌","十堰"];
provinces["四川"] = ["成都","内江","达州"];
provinces["河南"] =["郑州","南阳","信阳","漯河"];
function changeProvince()
{
var prov = document.getElementById("province").value;
var city =document.getElementById("city");
city.options.length =0;
for(var i in provinces[prov])
{
city.options.add(new Option(provinces[prov][i],provinces[prov][i]));
}
}
window.onload = function(){
var province = document.getElementById("province");
for(var index in provinces)
{
//alert(index);
province.options.add(new Option(index,index));
}
province.fireEvent("onchange");
};
</script>
</head>
<body>
省份:<select id="province" onchange= "changeProvince()"></select>
城市:<select id="city"></select>
</body>
</html>
