jQuery.attr() 源码解读

   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 === 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 === "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;
                }
            }
        }
    }