表单中分类选择

<form class="form" onload="init()">
                商品分类：
                <select name="commodity" id="province" onchange="selectProvince();">
                    <option selected="selected">&nbsp&nbsp&nbsp——请选择分类——&nbsp&nbsp&nbsp</option>
                    <option value="0">食品</option>
                    <option value="1">手机</option>
                    <option value="2">电脑</option>
                    <option value="3">日用品</option>
                    <option value="4">其他</option>
                </select>
                <br/>产品名称：
                <select name="name" id="city">
                </select>
                <br/>产品单价<br/>
                购买数量：<input type="text" id="quantity">&nbsp&nbsp&nbsp库存数量（最低一件）
            </form>

var provinces=[
            ["水果","零食","生鲜","蔬菜"],
            ["苹果","华为","三星","小米"],
            ["戴尔","惠普","微星","雷神"],
            ["牙刷","水杯","餐具","电池"],
            ["服装","电子","背包","游戏机"],
        ]
        function selectProvince(){
            var province=document.getElementById("province");
            var city=document.getElementById("city");
            var citys=provinces[province.value];
            city.options.length=0;
            for (var i=0;i<citys.length;i++) {
                var sel=document.createElement("option");
                var cityName=document.createTextNode(citys[i]);
                sel.appendChild(cityName);
                city.appendChild(sel);
            }
        }
        function init(){
            var province=document.getElementById("province");
            var city=document.getElementById("city");
            var citys=provinces[province.value];
            for (var i=0;i<citys.length;i++) {
                var sel=document.createElement("option");
                var cityName=document.createTextNode(citys[i]);
                sel.appendChild(cityName);
                city.appendChild(sel);
            }
        }