例如jQuery(“div#none”),chrome里面返回的是”[]”,也就是一个被jQuery封装过的对象

Returning an Empty Set
As of jQuery 1.4, calling the jQuery() method with no arguments returns an empty jQuery set (with a .length property of 0). In previous versions of jQuery, this would return a set containing the document node.

所以一般可以用jQuery(“div#none”).length是否大于0来判断某个元素是否存在