Javascript检查变量是否未定义

Question

To check whether adsense script is loaded or not, I use this:

var isAdsenseNotLoaded = (typeof adsbygoogle.loaded === 'undefined');

But from many users has this error in stack trace:

ReferenceError: adsbygoogle is not defined
    at http://example.com/file.js:1:42020

So should I also check whether adsbygoogle and also adsbygoogle.loaded ?

Answer 1

您需要检查是否adsbygoogle定义adsbygoogle ：

var isAdsenseNotLoaded = !adsbygoogle || typeof adsbygoogle.loaded === 'undefined';

Answer 2

Yes, check for typeof adsbygoogle first, this will return if the global variable adsbygoogle is loaded.

var isAdsenseNotLoaded = (typeof adsbygoogle === 'undefined' || typeof adsbygoogle.loaded === 'undefined');

Checking for global variables with typeof will never produce any exceptions due to trying to access an undefined variable. Reference: JavaScript check if variable exists (is defined/initialized)

Answer 3

So the whole object is not defined

var isAdsenseNotLoaded = (typeof adsbygoogle === 'undefined' || typeof adsbygoogle.loaded === 'undefined');

Just check in the first step if the object exists and then if it is loaded.