定义jquery对象，原型上添加方法
function _jQuery(selector){}
_jQuery.prototype={...}
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想更简洁的创建jQuery对象
jQuery=function(selector){return new _jQuery(selector);
}
function _jQuery(selector){}
创建一个快捷接口方便修改jQuery原型
jQuery.fn=_jQuery.prototype={...}
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将_jQuery挂到jQuery的原型属性init上
jQuery.prototype.init=_jQuery=function(selector){...
}
jQuery.fn=_jQuery.prototype={...}=jQuery.prototype.init.prototype
jQuery=function(selector){return new jQuery.prototype.init(selector);
}
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将_jQuery去掉
jQuery.prototype.init=function(selector){...
}
jQuery.fn={...}=jQuery.prototype.init.prototype
jQuery=function(selector){return new jQuery.prototype.init(selector);
}看到这里应该明白了平时我们用的最多jQuery($)只是个包装方法。
为了方便我们扩展实际的_jQuery对象(jQuery.prototype.init)原型暴露出$.fn,
在这基础上jQuery.fn=jQuery.prototype让我们用的更习惯。
所以最后出来这绕来绕去略显奇葩的源码：jQuery=function(selector){return new jQuery.prototype.init(selector);
}
jQuery.fn=jQuery.prototype={...init:function(){...}
}

jQuery.extend = jQuery.fn.extend = function() {var src, copyIsArray, copy, name, options, clone,target = arguments[0] || {},i = 1,length = arguments.length,deep = false;// Handle a deep copy situationif ( typeof target === "boolean" ) {deep = target;target = arguments[1] || {};// skip the boolean and the targeti = 2;}// Handle case when target is a string or something (possible in deep copy)if ( typeof target !== "object" && !jQuery.isFunction(target) ) {target = {};}// extend jQuery itself if only one argument is passedif ( length === i ) {target = this;--i;}for ( ; i < length; i++ ) {// Only deal with non-null/undefined valuesif ( (options = arguments[ i ]) != null ) {// Extend the base objectfor ( name in options ) {src = target[ name ];copy = options[ name ];// Prevent never-ending loopif ( target === copy ) {continue;}// Recurse if we're merging plain objects or arraysif ( deep && copy && ( jQuery.isPlainObject(copy) || (copyIsArray = jQuery.isArray(copy)) ) ) {if ( copyIsArray ) {copyIsArray = false;clone = src && jQuery.isArray(src) ? src : [];} else {clone = src && jQuery.isPlainObject(src) ? src : {};}// Never move original objects, clone themtarget[ name ] = jQuery.extend( deep, clone, copy );// Don't bring in undefined values} else if ( copy !== undefined ) {target[ name ] = copy;}}}}// Return the modified objectreturn target;
};