select 或text的onchange事件需要手动（通过键盘输入）改变select或text的值才能触发，如果在js中给select或text赋值，则无法触发onchang事件，
例如，在页面加载完成以后，需要触发一个onChange事件，在js中用document.getElementById("province").value="湖北";直接给select或text赋值是不行的，要想实现手动触发onchange事件，需要在js给select赋值后，加入下面的语句
document.getElementById("province").fireEvent('onchange') 来实现,
<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>