How to implement the corresponding callback function after loading using JS script

This article mainly explains how to execute the corresponding callback task after successfully loading the js file. Friends who are interested in the operation method of executing the corresponding callback function after the JS script is loaded can learn from this article.

Frequently used in projects You will encounter such a problem: when a js script is loaded, the corresponding task will be executed, but many friends may not know how to judge whether the js file we want to load has been loaded. If it is not loaded, we will call the js file. The function will not succeed. This article mainly explains how to execute the corresponding callback task after successfully loading the js file.

Basic idea

We can dynamically create the <script> element and then change its src attribute to load the script, but how do you know that the script file has been loaded? Because some functions need to be loaded before the script can be called. In IE browser, you can use onreadystatechange of element to monitor the change of loading state, and judge whether the script is loaded by judging 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, according to these two Simple principle, we make some modifications, I changed it into two functions, namely serial loading and parallel loading.

Serial and parallel dynamic scripts

When passing an array containing multiple JS file paths, the serial loading function starts from the first The loading of script files starts. Each time one is loaded successfully, the next script file starts to be loaded. 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 < scripts.length; i++) {
  s[i] = document.createElement(&#39;script&#39;);
  s[i].setAttribute(&#39;type&#39;,&#39;text/javascript&#39;);
  // Attach handlers for all browsers
  // 异步
  s[i].onload = s[i].onreadystatechange = function() {
   if(!/*@cc_on!@*/0 || this.readyState === &#39;loaded&#39; || this.readyState === &#39;complete&#39;) {
    loaded++;
    this.onload = this.onreadystatechange = null;
    this.parentNode.removeChild(this);
    if(loaded === scripts.length && typeof(callback) === &#39;function&#39;) callback();
   }
  };
  // 同步
  s[i].setAttribute(&#39;src&#39;,scripts[i]);
  HEAD.appendChild(s[i]);
 }
}

Here, the

<script> tag is dynamically inserted into the tag on the page, and after the loading is completed, the tag element will be Automatically removed.

Usage

Here is an array variable declared, which contains two remote JS file addresses (of course

< script> Tag calling script supports cross-domain):

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:'right', y:'bottom' },
  width: '480px',
  height: '50%',
  itempider : &#39;<hr>&#39;,
  listDOM : &#39;all&#39;
 });
 */
 console.log(&#39;脚本加载完成啦&#39;);
});

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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