这个就涉及到 jQuery 源码的问题了；

jquery 在代码尾部全局注册：
if ( !noGlobal ) {
window.jQuery = window.$ = jQuery;
}


// 这个是开始的代码，由于这个网站 module 是存在的，所以 noGlobal 为 true,
( function( global, factory ) {

"use strict";

if ( typeof module === "object" && typeof module.exports === "object" ) {

// For CommonJS and CommonJS-like environments where a proper `window`
// is present, execute the factory and get jQuery.
// For environments that do not have a `window` with a `document`
// (such as Node.js), expose a factory as module.exports.
// This accentuates the need for the creation of a real `window`.
// e.g. var jQuery = require("jquery")(window);
// See ticket #14549 for more info.
module.exports = global.document ?
factory( global, true ) :
function( w ) {
if ( !w.document ) {
throw new Error( "jQuery requires a window with a document" );
}
return factory( w );
};
} else {
factory( global );
}

// Pass this if window is not defined yet
} )( typeof window !== "undefined" ? window : this, function( window, noGlobal ) {...}