top:$elem.position().top报错Uncaught TypeError:Cannot read property 'top'of null 是什么原因?
var getValue=function(className){
$elem=$(''+className+'');
return {height:$elem.height(),
top:$elem.position().top};
};
top:$elem.position().top报错Uncaught TypeError:Cannot read property 'top'of null 是什么原因?