Google Analytics ID stored in cookie is undefined - cookies

Question

We are installing google analytics via Google Tag Manager.
We have custom variable that supposed to take the GA customer id, and send it to our GA.
The variable is defined as follows:
function() {
try {
var cookie = {{GA_ID_Cookie}}.split(".");
return cookie[2] + "." + cookie[3];
} catch(e) {
return 'N/A';
}
}
While {{GA_ID_Cookie}} is a first party cookie variable named "_ga".
In most cases, this values works, but there are some cases where GA_ID_Cookie is undefined (and exception is thrown).
It happens in all browsers. There enough users with "N/A", so its not about cookies disabled issue.
The GTM installs the GA on page view event; It uses this problematic variable as a custom dimension.
My question is how come the ga id is null, and how can we overcome this problem and get the id in other ways.

Answer 1

It is likely that your tag is fired before the cookie is generated.
Try to change the page view to window load. Clear the cookie and retry, it should work.

Answer 2

Like Ashley pointed out you might be facing a race condition whereby you try to access the cookie before it is set by GA.
Please note that the GA cookie ID contains some uninteresting info from the point of view of identifying users, namely the version which should be removed.
If your GA cookie looks like this:
_ga=GA1.2.1033501218.1368477899;
Then the part you're interested in is:
1033501218.1368477899
To retrieve the client ID via the browser, the official way is as follows:
https://developers.google.com/analytics/devguides/collection/analyticsjs/accessing-trackers
// Initializing the `ga` command queue so that commands
// can be queued even if the GA snippet is not loaded
window.ga=window.ga||function(){(ga.q=ga.q||[]).push(arguments)};ga.l=+new Date;
// Queuing a command to retrieve the Client ID when the tracker is ready
ga(function(tracker) {
// Logs the client ID for the current user.
console.log(tracker.get('clientId'));
});
If you are using GTM then you need to create a task:
https://www.simoahava.com/analytics/13-useful-custom-dimensions-for-google-analytics/#13-client-id
function() {
return function(model) {
return model.get('clientId');
};
}
If you want to retrieve the Client ID via the server, then you simply need to parse the cookie HTTP header (below example is from request to stackoverflow website) using an HTTP library of your choice and getting rid of the leading GA\d\.\d\. pattern which represents the cookie version.
cookie: prov=f67bae3b-f99c-2f22-84fc-7c2a62862f3d; _ga=GA1.2.1380536973.1571212618; ...