JavaScript array secondary linkage drop-down list

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<select id = "a1" onchange = "addOption()"></select>

<select id = "a2"></select>

<script type = "text/javascript">

var city = new Array;
city['Jiangsu'] = ['Nanjing','Shanghai', 'Lianyungang', 'Suzhou', 'Zhenjiang','Taizhou','Nantong','Huai'an'];
city[' Shandong'] = ['Qingdao','Yantai', 'Jinan','Qufu','Dezhou','Binzhou','Linyi','Heze'];
city['Hunan'] = ['Changsha', 'Zhuzhou','Xiangtan','Yueyang','Loudi','Changde','Huaihua','Zhangjiajie'];
function allCity() {
var select1 = document.getElementById("a1");
for(var i in city) { //Pay attention to the way of traversing the array here
select1.add(new Option(i, i), null);
}
addOption(); //Initialization options
}
function addOption() {
var select2 = document.getElementById(" a2");
var select1 = document.getElementById("a1").value;
select2.length = 0; //Clear the city menu every time
if(select1 != 'Please select a province') {
for(var i in city[select1 ]) {
select2.add(new Option(city[select1][i], city[select1][i]), null);
}
}else if (sheng == 'Please select a province'){
select2.length = 0;
select2. add(new Option("Please select a city", "Please select a city"), null);
}
}
window.onload = allCity();

</script>

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