attr: function( elem, name, value ) {        var hooks, ret,            nType = elem.nodeType;        //如果elem不存在，或者是文本、注释、属性节点        // don‘t get/set attributes on text, comment and attribute nodes        if ( !elem || nType === 3 || nType === 8 || nType === 2 ) {            return;        }        // Fallback to prop when attributes are not supported        if ( typeof elem.getAttribute === core_strundefined ) {//如果elem不支持getAttribute 比如document或者文档碎片            return jQuery.prop( elem, name, value );//调用jQuery.prop方法        }        // All attributes are lowercase        // Grab necessary hook if one is defined        //？        if ( nType !== 1 || !jQuery.isXMLDoc( elem ) ) {//elem不是标签或者elem不在xml中            name = name.toLowerCase();//属性名大写            hooks = jQuery.attrHooks[ name ] ||                ( jQuery.expr.match.bool.test( name ) ? boolHook : nodeHook );//jQuery.attrHooks.type        }        if ( value !== undefined ) {//赋值            if ( value =http://www.mamicode.com/== null ) {//如果设的值为null，相当于移除attr ,比如.attr(‘checked‘,null);                jQuery.removeAttr( elem, name );            } else if ( hooks && "set" in hooks && (ret = hooks.set( elem, value, name )) !== undefined ) {//调用钩子的set方法                return ret;            } else {                elem.setAttribute( name, value + "" );//如果value是数值，隐式转为字符串                return value;            }        } else if ( hooks && "get" in hooks && (ret = hooks.get( elem, name )) !== null ) {//调用钩子的get            return ret;        } else {//取值            ret = jQuery.find.attr( elem, name );//调用Sizzle.attr  因为jquery中有一句：jQuery.find = Sizzle;            //将null值修正为undefined            // Non-existent attributes return null, we normalize to undefined            return ret == null ?                undefined :                ret;        }    }

    Sizzle.attr = function( elem, name ) {    // Set document vars if needed    if ( ( elem.ownerDocument || elem ) !== document ) {//如果不是当前document        setDocument( elem );//不知道干嘛 考虑iframe的情况？    }    //Expr.attrHandle是一个对象，包含了    //async autofocus autoplay checked controls defer disabled hidden ismap loop     //multiple open readonly required scoped selected 等自定义方法(属性)    var fn = Expr.attrHandle[ name.toLowerCase() ],        // Don‘t get fooled by Object.prototype properties (jQuery #13807)        //hasOwn 就是hasOwnProperty         //所以这个hasOwn.call( Expr.attrHandle, name.toLowerCase() ) 是在判断传入的属性名是不是这个对象的        //自定义属性，而不是原型上的属性        //val为调用自定义方法的结果或者undefined        val = fn && hasOwn.call( Expr.attrHandle, name.toLowerCase() ) ?            fn( elem, name, !documentIsHTML ) :            undefined;    //如果val为undefined        //如果（support.attributes为真或者document不是html），返回elem.getAttribute( name )的值        //否则 val为 elem.getAttributeNode(name))的值            //如果val为真，并且val.specified为真，返回val.value            //否则返回null    //否则返回val    return val === undefined ?        support.attributes || !documentIsHTML ?            elem.getAttribute( name ) :            (val = elem.getAttributeNode(name)) && val.specified ?                val.value :                null :        val;};

    attrHooks: {        type: {            set: function( elem, value ) {                if ( !jQuery.support.radioValue && value =http://www.mamicode.com/=="radio" && jQuery.nodeName(elem, "input") ) {                    // Setting the type on a radio button after the value resets the value in IE6-9                    // Reset value to default in case type is set after value during creationvar val = elem.value;                    elem.setAttribute( "type", value );                    if ( val ) {                        elem.value = val;                    }                    return value;                }            }        }    }