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How to dynamically load js to improve web page opening speed_javascript skills

WBOYOriginal: 2016-05-16 16:42:311044browse

Generally speaking, if you load all the required JavaScript code at once, it will cause the initial web page to open slowly. However, many of the loaded codes do not need to be used. This unnecessary waste of performance should be avoided. If you want to dynamically load JavaScript code, you can use the DOM model to add a 3f1c4e4b6b16bbbd69b2ee476dc4f83a node in the HTML document, and set the src attribute of this node (i.e., external Javascript file) to the JavaScript code that needs to be dynamically loaded.

The following is an example of completing such a function:

(1), create a new JsLoaderTest.html file

<html xmlns="http://www.w3.org/1999/xhtml"> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" /> 
<title>按需载入JavaScript代码的例子</title> 
<script type="text/javascript"> 
  function JsLoader(){ 
  this.load=function(url){ 
      //获取所有的<script>标记 
      var ss=document.getElementsByTagName("script"); 
      //判断指定的文件是否已经包含，如果已包含则触发onsuccess事件并返回 
      for (i=0;i<ss.length;i++){ 
        if (ss[i].src && ss[i].src.indexOf(url)!=-1){ 
          this.onsuccess(); 
          return; 
        } 
      } 
      //创建script结点,并将其属性设为外联JavaScript文件 
      s=document.createElement("script"); 
      s.type="text/javascript"; 
      s.src=url; 
      //获取head结点，并将<script>插入到其中 
      var head=document.getElementsByTagName("head")[0]; 
      head.appendChild(s); 
      //获取自身的引用 
      var self=this; 
      //对于IE浏览器，使用readystatechange事件判断是否载入成功 
      //对于其他浏览器，使用onload事件判断载入是否成功 
      //s.onload=s.onreadystatechange=function(){ 
      s.onload=s.onreadystatechange=function(){ 
        //在此函数中this指针指的是s结点对象，而不是JsLoader实例, 
        //所以必须用self来调用onsuccess事件，下同。 
        if (this.readyState && this.readyState=="loading") return; 
        self.onsuccess(); 
      } 
      s.onerror=function(){ 
        head.removeChild(s); 
        self.onfailure(); 
      } 
    }; 
    //定义载入成功事件 
    this.onsuccess=function(){}; 
    //定义失败事件 
    this.onfailure=function(){}; 
  } 
  function btnClick(){ 
      //创建对象 
    var jsLoader=new JsLoader(); 
    //定义载入成功处理程序 
    jsLoader.onsuccess=function(){ 
       sayHello(); 
    } 
    //定义载入失败处理程序 
    jsLoader.onfailure=function(){ 
       alert("文件载入失败！"); 
    } 
    //开始载入 
    jsLoader.load("hello.js"); 
  } 
</script> 
</head> 
<body> 
<label> 
<input type="submit" name="Submit" onClick="javascript:btnClick()" value="载入JavaScript文件"> 
</label> 
</body> 
</html>

(2). Create a new hello.js file, including the following code:

// JavaScript Document 
function sayHello(){ 
  alert("Hello World!成功载入JavaScript文件");   
} 
// JavaScript Document
function sayHello(){
  alert("Hello World!成功载入JavaScript文件");  
}

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