Analysis on the implementation principle of jQuery's ready method

The ready method in jQuery achieves the effect of being executed only after the page is loaded, but it is not a package of window.onload or document.onload, but is completed using the standard W3C browser DOM to hide the API and IE browser defects. , first of all, let’s look at the jQuery code

DOMContentLoaded = function()
 {
  //取消事件监听，执行ready方法
 if ( document.addEventListener )
 {  
  document.removeEventListener( "DOMContentLoaded", DOMContentLoaded, false );
  jQuery.ready();
 }
  else if ( document.readyState === "complete" )
 {
  document.detachEvent( "onreadystatechange", DOMContentLoaded );
  jQuery.ready();
 }
};

jQuery.ready.promise = function( obj ) {
 if ( !readyList ) {
 
  readyList = jQuery.Deferred();
   //表示页面已经加载完成，直接调用 ready方法
  if ( document.readyState === "complete" ) {
   //将 jQuery.ready压入异步消息队列，设置延迟时间1毫秒（注意，有些浏览器延迟不能小于4毫秒）
   setTimeout( jQuery.ready);
  }
  else if ( document.addEventListener ) //
  {
    //监听DOM加载完成
   document.addEventListener( "DOMContentLoaded", DOMContentLoaded, false );
    //这里是为了确保所有ready执行结束，如果DOMContentLoaded方法执行了，将有一个状态值 isReady被设置为true,因此，
    //ready方法一旦执行，那么将只执行一次，window.addEventListener中的ready 将被 return 中断
   window.addEventListener( "load", jQuery.ready, false );
  
  } else {
   //低版本的IE浏览器
   document.attachEvent( "onreadystatechange", DOMContentLoaded );
   window.attachEvent( "onload", jQuery.ready );
 
   var top = false;
 
   try {
    top = window.frameElement == null && document.documentElement;
   } catch(e) {}
 
   if ( top && top.doScroll ) //剔除iframe的成分
   {
    (function doScrollCheck() {
     if ( !jQuery.isReady ) {
 
      try {
       //根据bug来兼容低版本的IE http://javascript.nwbox.com/IEContentLoaded/
       top.doScroll("left");
      } catch(e) {
       //由于低版本的IE 浏览器，onreadystatechange事件不可靠，因此需要根据各个bug来判断页面是否已加载完成
       return setTimeout( doScrollCheck, 50 );
      }
 
      jQuery.ready();
     }
    })();
   }
  }
 }
 return readyList.promise( obj );
};

ready: function( wait )
 {
 
 if ( wait === true ? --jQuery.readyWait : jQuery.isReady ) {
  //判断页面是否已完成加载并且是否已经执行ready方法
  return;
 }
 
 
 if ( !document.body ) {
  return setTimeout( jQuery.ready );
 }
 
   
 jQuery.isReady = true; //指示ready方法已被执行
 
   
 if ( wait !== true && --jQuery.readyWait > 0 ) {
  return;
 }
 
   
 readyList.resolveWith( document, [ jQuery ] );
 
   
 if ( jQuery.fn.trigger ) {
  jQuery( document ).trigger("ready").off("ready");
 }
},