Jquery获取checkbox属性checked为undefined

As of jQuery 1.6, the .attr() method returns undefined for attributes that have not been set. In addition, .attr() should not be used on plain objects, arrays, the window, or the document. To retrieve and change DOM properties, use the .prop()method.

v1.6以后attr(‘checked’)就返回checked和undefined,

v1.6以前返回true和false,

v1.6以后可以使用is(':checked')或者.prop('checked')来返回true和false