Write Google Analytics Client ID to localStorage

gacode-example-gtag-js.html

<!--
Replace existing standard gtag.js tracking-code with this
snippet in order to store the Client ID in localStorage and use this ID when present

converted for direct code implementation from Simo Ahava´s solution for GTM; see 
https://www.simoahava.com/analytics/use-localstorage-client-id-persistence-google-analytics/
for details

Client IDs will survive deletion of cookies by user or ITP 2.1

 - replace UA-xxxxx-y with your own property id  (2 instances)
 - adjust special configuration like custom dimensions if present in existing code
 - tracker is initialized with option 'anonymize_ip': true. Delete (2 instances) if not needed 
 - you do not need this comment or the other comments in the code below in a live source code - delete them after implementation!

NOTE: this solution only works for single-host domains like www.domain.com. 
If you measure multiple subdomains like blog.domain.com, www.domain.com, app.domain.com 
in a single property, localStorage is no solution! 
-->

<script async src="https://www.googletagmanager.com/gtag/js?id=UA-xxxxx-y"></script>
<script>
var GA_TRACKING_ID = 'UA-xxxxx-y';
var localStorageCid = {
    objectName: 'ga_client_id',
    expires: 1000*60*60*24*365*2
  };
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());

if (window.localStorage) {
  var lsCid;
  var jsonObj = window.localStorage.getItem(localStorageCid.objectName) || '{}';
  var obj = JSON.parse(jsonObj);
  var now = new Date().getTime();
  if (obj.clientId && obj.expires) {
    if (now <= obj.expires) {
      lsCid = obj.clientId;
    }
  }
  
  //initialize - but do not send auto-hit
  //add your own additional options here if present in existing code
  gtag('config', GA_TRACKING_ID, { 'anonymize_ip': true, 'send_page_view': false, 'client_id': lsCid});
  
  //pageview gets sent now with callback for storing the Client ID 
  gtag('event', 'page_view', {
    'event_callback': function() {
      var tracker_id = GA_TRACKING_ID.replace(/-/g, '_');
      var tracker = window.ga.getByName('gtag_'+tracker_id);
      var _lsc_clientId = tracker.get('clientId');
      var _lsc_obj = JSON.stringify({
          clientId: _lsc_clientId,
          expires: new Date().getTime() + localStorageCid.expires
      });
      window.localStorage.setItem(localStorageCid.objectName, _lsc_obj); 
    }
  });
}
else {
  //add your own additional options here if present in existing code
  gtag('config', GA_TRACKING_ID, { 'anonymize_ip': true });
}  
</script>

gacode-example-universal.html

<!--
Replace existing standard Universal Analytics tracking-code with this
snippet in order to store the Client ID in localStorage and use this ID when present

converted for direct code implementation from Simo Ahava´s solution for GTM; see 
https://www.simoahava.com/analytics/use-localstorage-client-id-persistence-google-analytics/
for details

Client IDs will survive deletion of cookies by user or ITP 2.1

 - replace UA-xxxxx-y with your own property id
 - adjust special configuration like custom dimensions below if present in existing code
 - you do not need this comment or the other comments in the code below in a live source code - delete them after implementation!

NOTE: this solution only works for single-host domains like www.domain.com. 
If you measure multiple subdomains like blog.domain.com, www.domain.com, app.domain.com 
in a single property, localStorage is no solution! 
-->
<script>
var GA_TRACKING_ID = 'UA-xxxxx-y';
var localStorageCid = {
    objectName: 'ga_client_id',
    expires: 1000*60*60*24*365*2
  };
(function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
(i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
})(window,document,'script','https://www.google-analytics.com/analytics.js','ga');

if (window.localStorage) {
  var lsCid;
  var jsonObj = window.localStorage.getItem(localStorageCid.objectName) || '{}';
  var obj = JSON.parse(jsonObj);
  var now = new Date().getTime();
  if (obj.clientId && obj.expires) {
    if (now <= obj.expires) {
      lsCid = obj.clientId;
    }
  }

  ga('create', GA_TRACKING_ID, { 'clientId': lsCid });
  ga(function(tracker) {
    var _lsc_clientId = tracker.get('clientId');
    var _lsc_obj = JSON.stringify({
      clientId: _lsc_clientId,
      expires: new Date().getTime() + localStorageCid.expires
    });
    window.localStorage.setItem(localStorageCid.objectName, _lsc_obj);
  });
}
else {
  ga('create', GA_TRACKING_ID, 'auto');
}

//Comment out if no anonymization required   
ga('set', 'anonymizeIp', true);
//add additional configuration here. Example
//ga('set', 'dimension1', 'foo');  
ga('send', 'pageview');  
</script>

Thanks!