JS对外部文件的加载及对IFRMAME的加载的实现,当加载完成后,指定指向方法(方法回调)

JS对外部文件的加载：

1. 使用
   1. 使用XMLHttpRequest对象异步加载JS文件

   function loadScript(url, callback) {
   var script = document.createElement('script');
   script.type = 'text/javascript';
   if (script.readyState) { //IE
   script.onreadystatechange = function() {
   if (script.readyState == "loaded" || script.readyState == "complete") {
   script.onreadystatechange = null;
   callback();
   }
   };
   } else { //Others
   script.onload = function() {
   callback();
   };
   }
   script.src = url;
   document.getElementsByTagName("head")[0].appendChild(script);
   }

   loadScript("path/to/external/script.js", function() {
   console.log("Script loaded");
   });

   IFRAME的加载：

   1. 通过JS创建IFRAME元素

   var iframe = document.createElement('iframe');
   iframe.src = 'path/to/external/iframe.html';

   1. 修改IFRAME的src属性

   var iframe = document.getElementById('myIframe');
   iframe.src = 'path/to/external/iframe.html';

   当加载完成后，指定指向方法的实现：

   1. 使用传统的回调方式：

   function loadScript(url, callback) {
   var script = document.createElement('script');
   script.type = 'text/javascript';
   if (script.readyState) { //IE
   script.onreadystatechange = function() {
   if (script.readyState == "loaded" || script.readyState == "complete") {
   script.onreadystatechange = null;
   callback();
   }
   };
   } else { //Others
   script.onload = function() {
   callback();
   };
   }
   script.src = url;
   document.getElementsByTagName("head")[0].appendChild(script);
   }

   loadScript("path/to/external/script.js", function() {
   console.log("Script loaded");
   });

   1. 使用Promise方式：

   function loadScript(url) {
   return new Promise(function(resolve, reject) {
   var script = document.createElement('script');
   script.type = 'text/javascript';
   script.onload = resolve;
   script.onerror = reject;
   script.src = url;
   document.getElementsByTagName("head")[0].appendChild(script);
   });
   }

   loadScript("path/to/external/script.js")
   .then(function() {
   console.log("Script loaded");
   })
   .catch(function() {
   console.log("Error loading script");
   });

   示例说明：

   1. 加载外部JS文件并指定回调方法的示例：

   function loadScript(url, callback) {
   var script = document.createElement('script');
   script.type = 'text/javascript';
   if (script.readyState) { //IE
   script.onreadystatechange = function() {
   if (script.readyState == "loaded" || script.readyState == "complete") {
   script.onreadystatechange = null;
   callback();
   }
   };
   } else { //Others
   script.onload = function() {
   callback();
   };
   }
   script.src = url;
   document.getElementsByTagName("head")[0].appendChild(script);
   }

   loadScript("https://code.jquery.com/jquery-latest.min.js", function() {
   console.log("jQuery loaded");
   });

   1. 加载IFRAME并指定回调方法的示例：

   var iframe = document.createElement('iframe');
   iframe.src = 'https://www.baidu.com';
   iframe.onload = function() {
   console.log("IFRAME loaded");
   };
   document.body.appendChild(iframe);