JS中只有数组，没有集合
数组长度=角标+1 长度可变
cities[0],cities[1],cities[2],cities[3]
cities[100] 长度是101 而不是5
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<script>
//1.创建一个二维数组用于存储省份和城市
var cities = new Array(3);
cities[0] = new Array("武汉市","黄冈市","襄阳市","荆州市");
cities[1] = new Array("长沙市","郴州市","株洲市","岳阳市");
cities[2] = new Array("石家庄市","邯郸市","廊坊市","保定市");
cities[3] = new Array("郑州市","洛阳市","开封市","安阳市");
function changeCity(val){
//7.获取第二个下拉列表
var cityEle = document.getElementById("city");
//9.清空第二个下拉列表的option内容
cityEle.options.length=0;
//2.遍历二维数组中的省份
for(var i=0;i<cities.length;i++){
if(val==i){
for(var j=0;j<cities[i].length;j++){
var textNode=document.createTextNode(cities[i][j]);
var opEle = document.createElement("option");
opEle.appendChild(textNode);
cityEle.appendChild(opEle);
}
}
}
}
</script>
</head>
<body>
<form action="#">
<select onchange="changeCity(this.value)">
<option>--请选择--</option>
<option value="0">湖北</option>
<option value="1">湖南</option>
<option value="2">河北</option>
<option value="3">河南</option>
</select>
<select id="city">
</select>
</form>
</body>
</html>