JavaScript实现二级联动下拉框

html代码段
<select id="province" onchange="changeCity(this.value)">
<option value="0">----请选择省份----</option>
<option value="1">山西省</option>
<option value="2">陕西省</option>
<option value="3">四川省</option>
<option value="4">浙江省</option>
</select>
<select id="city">
<option value="">----请选择城市----</option>
</select>

js代码段
<script>
function $(id){
return document.getElementById(id);
}
var arrCity = [
["----请选择城市----"],
["太原市","大同市","朔州市","运城市"],
["西安市","韩城市","咸阳市","临潼市"],
["成都市","绵阳市","攀枝花","广元市"],
["杭州市","温州市","丽水市","金华市"]
];

function changeCity(index) {
var citys = arrCity[index];
var city = $("city");
city.length = 0;
for (var i = 0; i < citys.length; i++) {
var opt = document.createElement("option");
opt.innerHTML = citys[i];
city.appendChild(opt);
}
}
</script>