quietlynn/google-plus-extension-jsapi — comment on commit 45b174fb0fbaef404c7846f7c2e92d6d0608cd7b
File: jsapi/jsapi_for_google_plus.js line 294
By mohamedmansour:

Getting the profile ID is already implemented :)

this.refreshInfo();

So what you should do is that, when we init() this API, you should call refreshInfo() directly, then you can get the ID by doing this:

this.getInfo().id;