JQ核心骨架

(function(){

    var jQuery = function(id){
        return new _jquery(id);
    };

    var _jquery = function(id){
        //此处各种选择分支神马的都忽略～
        this[0] = document.getElementById(id);
        this.length = 1;
    };

    jQuery.fn = jQuery.prototype = {
        constructor: jQuery,
        addClass: function(className){
            this[0].className += ' ' + className;
        }
    };

    _jquery.prototype = jQuery.fn;

    window.$ = window.jQuery = jQuery;

})();

(function( window, undefined ) {
var
    jQuery = function( selector, context ) {
        // The jQuery object is actually just the init constructor 'enhanced'
        return new jQuery.fn.init( selector, context, rootjQuery );
    }；

    //各种原型属性
    jQuery.fn = jQuery.prototype = {
        constructor: jQuery,
        init: function( selector, context, rootjQuery ) {
            //...
        },
        ...
    };
    jQuery.fn.init.prototype = jQuery.fn;

    //extend方法，用来扩展jQuery，或jQuery.fn
    jQuery.extend = jQuery.fn.extend = function() {
        //...
    };

    jQuery.fn.extend({
        addClass: function( value ) {
            //...
            return this;    //返回this，链式调用的秘密
        }
    });

    window.jQuery = window.$ = jQuery;

})( window );