var    // A central reference to the root jQuery(document)
    rootjQuery,// The deferred used on DOM ready
    readyList;jQuery.ready.promise = function( obj ) {if ( !readyList ) { //第一次readyList为空可以进来，后续就进不来if了，只执行一次
readyList = jQuery.Deferred(); //第一步，创建延迟对象if ( document.readyState === "complete" ) { //DOM加载完成的标志就是document.readyState为complete，如果DOM已经加载好了就直接调工具方法jQuery.ready。setTimeout( jQuery.ready );//加定时器是为了兼容IE} else {//DOM没有加载完检测，即检测了DOMContentLoaded事件，也检测了load事件；最终走回调completed函数// Use the handy event callbackdocument.addEventListener( "DOMContentLoaded", completed, false );// A fallback to window.onload, that will always workwindow.addEventListener( "load", completed, false ); //因为火狐浏览器会缓存load事件，为了第一时间相应所以对load也监听了
        }}return readyList.promise( obj );
};

    // The ready event handler and self cleanup methodcompleted = function() {//不管是DOMContentLoaded事件还是load发生，都会取消2个事件监听//jQuery.ready()只会触发一次document.removeEventListener( "DOMContentLoaded", completed, false );window.removeEventListener( "load", completed, false );jQuery.ready();};

   $(function(arg){alert(this); //[object HTMLDocument]alert(arg); //jQuery函数})

<script>
$(document).on("ready",function(){alert(123); //123
});
</script>

// Handle when the DOM is ready
ready: function( wait ) { //参数和holdReady有关// Abort if there are pending holds or we're already readyif ( wait === true ? --jQuery.readyWait : jQuery.isReady ) {return;}// Remember that the DOM is readyjQuery.isReady = true;// If a normal DOM Ready event fired, decrement, and wait if need beif ( wait !== true && --jQuery.readyWait > 0 ) {return;}//第一步看这里重点，resolveWith改变状态的时候传参了，给done中方法fn传入了参数，//document是fn的this指向,jQuery是参数// If there are functions bound, to execute
    readyList.resolveWith( document, [ jQuery ] );//跟主动触发有关// Trigger any bound ready eventsif ( jQuery.fn.trigger ) {jQuery( document ).trigger("ready").off("ready");}
},

$.holdReady(true);
$(function () {alert(123); //调用了holdReady并传参true,就不能弹出123了
});

$.holdReady(true);
$.holdReady(false);
$(function () {alert(123); //释放了holdReady,就弹出123
});

$.getScript('js/a.js',function(){ //异步加载，不会影响后续代码执行。可能会产生一个问题，alert(2)先执行了a.js还没有加载完

})$(function () {alert(2);//先弹2，后弹出1
});

    $.holdReady(true); //在这里先hold住$.getScript('js/a.js', function () { //异步加载，不会影响后续代码执行。可能会产生一个问题，alert(2)先执行了a.js还没有加载完$.holdReady(false); //加载完释放，不hold了就可以弹2了
    })$(function () {alert(2);//先弹2，后弹出1});

// Is the DOM ready to be used? Set to true once it occurs.
isReady: false,
// A counter to track how many items to wait for before
// the ready event fires. See #6781
readyWait: 1,//第二步看这里
//holdReady推迟DOM的触发
// Hold (or release) the ready event
holdReady: function( hold ) {if ( hold ) {jQuery.readyWait++;//hold为真，让readyWait加加处理} else {jQuery.ready( true );}
},// Handle when the DOM is ready
ready: function( wait ) { //参数和holdReady有关// Abort if there are pending holds or we're already ready//readyWait为0时就不用hold了，执行后面的操作if ( wait === true ? --jQuery.readyWait : jQuery.isReady ) { return;}// Remember that the DOM is readyjQuery.isReady = true; //默认是false// If a normal DOM Ready event fired, decrement, and wait if need beif ( wait !== true && --jQuery.readyWait > 0 ) {return;}//第一步看这里重点，resolveWith改变状态的时候传参了，给done中方法fn传入了参数，//document是fn的this指向,jQuery是参数// If there are functions bound, to execute
    readyList.resolveWith( document, [ jQuery ] );//跟主动触发有关// Trigger any bound ready eventsif ( jQuery.fn.trigger ) {jQuery( document ).trigger("ready").off("ready");}
},