在控制台中获取未定义的错误通知，即使我正在检查未定义

Question

This code:

if(typeof(Listings.page_num) != 'undefined') {
   // something
}

Triggers this notice in firebug console:

Listings is not defined http://mysite.com/js/content_ads.js Line 13

Well duh, that's why I checked! Is there any way to get rid of this notice?

Answer 1

Do this.

var Listings = Listings || {};
if (typeof Listings.page_num !== "undefined") // page_num isset