Method to execute JS callback function after script is loaded

This time I will show you how to execute the JScallback function after the script is loaded. Notes on executing the JS callback function after the script is loaded What are they? Here are actual cases. Let’s take a look.

We often encounter this problem in projects: when a certain js script is loaded, the corresponding task will be executed, but many friends may not know how to judge the js we want to load. Whether the file has been loaded. If it is not loaded, calling the function in the js file will not be successful. This article mainly explains how to execute the corresponding callback task after successfully loading the js file.

The basic idea

We can dynamically create the <script></script> element and then load the script by changing its src attribute, but how do we know that the script file has been loaded? Because some functions need to be loaded before the script can be called. In the IE browser, you can use the onreadystatechange of the element to monitor changes in the loading state, and determine whether the script is loaded by determining whether its readyState is loaded or complete. Non-IE browsers can use onload to directly determine whether the script is loaded.

Simple example of dynamic script

A simple implementation process is as follows:

// IE下：
var HEAD = document.getElementsByTagName('head')[0] || document.documentElement
var src = 'http://xxxxxx.com'
var script = document.createElement('script')
script.setAttribute('type','text/javascript')
script.onreadystatechange = function() {
 if(this.readyState === 'loaded' || this.readyState === 'complete') {
  console.log('加载成功！')
 }
}
script.setAttribute('src', src)
HEAD.appendChild(script)
// Chrome等现代浏览器：
var HEAD = document.getElementsByTagName('head')[0] || document.documentElement;
var src = 'http://xxxxxx.com'
var script = document.createElement('script')
script.setAttribute('type','text/javascript')
script.onload = function() {
 console.log('加载成功!')
}
script.setAttribute('src', src)
HEAD.appendChild(script)

The principle is very simple. Based on these two simple principles, we made some modifications. I changed them into two functions, namely serial loading and parallel loading.

Serial and parallel dynamic scripts

When an array containing multiple JS file paths is passed, the serial loading function starts from loading the first script file, and starts loading the next script file after each successful loading. After all loading is completed, the callback function is executed. Parallel loading loads all script files from the beginning, that is, they start loading from the same point. When all loading is completed, the callback function is executed.

/** 
 * 串行加载指定的脚本
 * 串行加载[异步]逐个加载，每个加载完成后加载下一个
 * 全部加载完成后执行回调
 * @param {Array|String} scripts 指定要加载的脚本
 * @param {Function} callback 成功后回调的函数
 * @return {Array} 所有生成的脚本元素对象数组
 */
function seriesLoadScripts(scripts, callback) {
 if(typeof(scripts) !== 'object') {
  var scripts = [scripts];
 }
 var HEAD = document.getElementsByTagName('head')[0] || document.documentElement;
 var s = [];
 var last = scripts.length - 1;
 //递归
 var recursiveLoad = function(i) {
  s[i] = document.createElement('script');
  s[i].setAttribute('type','text/javascript');
  // Attach handlers for all browsers
  // 异步
  s[i].onload = s[i].onreadystatechange = function() {
   if(!/*@cc_on!@*/0 || this.readyState === 'loaded' || this.readyState === 'complete') {
    this.onload = this.onreadystatechange = null; 
    this.parentNode.removeChild(this);
    if(i !== last) {
     recursiveLoad(i + 1);
    } else if (typeof(callback) === 'function') {
     callback()
    };
   }
  }
  // 同步
  s[i].setAttribute('src', scripts[i]);
  HEAD.appendChild(s[i]);
 };
 recursiveLoad(0);
}
/**
 * 并行加载指定的脚本
 * 并行加载[同步]同时加载，不管上个是否加载完成，直接加载全部
 * 全部加载完成后执行回调
 * @param {Array|String} scripts 指定要加载的脚本
 * @param {Function} callback 成功后回调的函数
 * @return {Array} 所有生成的脚本元素对象数组
 */ 
function parallelLoadScripts(scripts, callback) {
 if(typeof(scripts) !== 'object') {
  var scripts = [scripts];
 }
 var HEAD = document.getElementsByTagName('head')[0] || document.documentElement;
 var s = [];
 var loaded = 0;
 for(var i = 0; i 
Here, the <p style="text-align: left;"><script> <code> tag is dynamically inserted into the <head> <code> tag on the page, and the tag element will be automatically removed after the loading is completed. </script></p>
	<p style="text-align: left;"><span style="color:#ff0000;">Instructions<strong></strong></span></p>
An array variable is declared here, which contains two remote JS file addresses (of course <p style="text-align: left;"><script><code> tag calling script supports cross-domain): <pre class="brush:php;toolbar:false">var scripts = [ 
 "http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js",
 "http://wellstyled.com/files/jquery.debug/jquery.debug.js"
];
// 这两个文件分别是 jQuery 1.4.的库文件和 jQuery Debug 插件
// 然后你可以使用下面的方法调用并在成功后执行回调了。
parallelLoadScripts(scripts, function() { 
 /*
 debug = new $.debug({ 
  posTo : { x:&#39;right&#39;, y:&#39;bottom&#39; },
  width: &#39;480px&#39;,
  height: &#39;50%&#39;,
  itempider : &#39;<hr>&#39;,
  listDOM : &#39;all&#39;
 });
 */
 console.log(&#39;脚本加载完成啦&#39;);
});</script></p>

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on php Chinese website!

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