Secondary linkage effect of drop-down box implemented by js_javascript skills

The example in this article describes the secondary linkage effect of the drop-down box implemented by js. Share it with everyone for your reference, the details are as follows:

<script language="JavaScript" type="text/javascript">
<!--
/*
 * 说明：将指定下拉列表的选项值清空
 * 转自：Gdong Elvis ( http://www.gdcool.net )
 *
 * @param {String || Object]} selectObj 目标下拉选框的名称或对象，必须
 */
 function removeOptions(selectObj)
 {
 if (typeof selectObj != 'object')
 {
 selectObj = document.getElementById(selectObj);
 }
 // 原有选项计数
 var len = selectObj.options.length;
 for (var i=0; i < len; i++) {
 // 移除当前选项
 selectObj.options[0] = null;
 }
 }
 /*
 * @param {String || Object]} selectObj 目标下拉选框的名称或对象，必须
 * @param {Array} optionList 选项值设置 格式：[{txt:'北京', val:'010'}, {txt:'上海', val:'020'}] ，必须
 * @param {String} firstOption 第一个选项值，如：“请选择”，可选，值为空
 * @param {String} selected 默认选中值，可选
 */
 function setSelectOption(selectObj, optionList, firstOption, selected) {
 if (typeof selectObj != 'object')
 {
 selectObj = document.getElementById(selectObj);
 }
 // 清空选项
 removeOptions(selectObj);
 // 选项计数
 var start = 0;
 // 如果需要添加第一个选项
 if (firstOption) {
 selectObj.options[0] = new Option(firstOption, '');
 // 选项计数从 1 开始
 start ++;
 }
 var len = optionList.length;
 for (var i=0; i < len; i++) {
 // 设置 option
 selectObj.options[start] = new Option(optionList[i].txt, optionList[i].val);
 // 选中项
 if(selected == optionList[i].val)  {
 selectObj.options[start].selected = true;
 }
 // 计数加 1
 start ++;
 }
 }
 //-->
</script>
<script language="JavaScript" type="text/javascript">
var cityArr = [];
cityArr['江苏省'] =
[
 {txt:'南京', val:'南京'},
 {txt:'无锡', val:'无锡'},
 {txt:'徐州', val:'徐州'},
 {txt:'苏州', val:'苏州'},
 {txt:'南通', val:'南通'},
 {txt:'淮阴', val:'淮阴'},
 {txt:'扬州', val:'扬州'},
 {txt:'镇江', val:'镇江'},
 {txt:'常州', val:'常州'}
 ];
cityArr['浙江省'] =
[
 {txt:'杭州', val:'杭州'},
 {txt:'宁波', val:'宁波'},
 {txt:'温州', val:'温州'},
 {txt:'湖州', val:'湖州'}
 ];
function setCity(province)
{
 setSelectOption('city', cityArr[province], '-请选择-');
}
</script>
 <select name="province" id="province" onchange="if(this.value != '') setCity(this.options[this.selectedIndex].value);">
 <option value="">-请选择-</option>
 <option value="江苏省">江苏省</option>
 <option value="浙江省">浙江省</option>
 </select>
 省
 <select name="city" id="city">
 <option value="">-请选择-</option>
 </select>
 市

PS: Here we recommend a very easy-to-use JavaScript compression, formatting and encryption tool with very powerful functions:

JavaScript compression/formatting/encryption tool: http://tools.jb51.net/code/jscompress

The encryption function in the above js tool can realize the eval function encryption form of js code. In this regard, this site also provides the following decryption tool for eval function encryption, which is very powerful and practical!

JS eval method online encryption and decryption tool: http://tools.jb51.net/password/evalencode

For more JavaScript-related content, please view the special topics on this site: "Summary of JavaScript switching special effects and techniques", "Summary of JavaScript search algorithm techniques", "JavaScript animation Summary of special effects and techniques", "Summary of JavaScript errors and debugging techniques", "Summary of JavaScript data structures and algorithm techniques", "Summary of JavaScript traversal algorithms and techniques " and "Summary of JavaScript mathematical operation usage"

I hope this article will be helpful to everyone in JavaScript programming.