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>

Does this replace cookie storage with localStorage? Or does it use localStorage as a backup if cookies were deleted?

Does this replace cookie storage with localStorage? Or does it use localStorage as a backup if cookies were deleted?

It uses localStorage instead of cookies. So with later iterations of ITP, this makes no difference any longer. For ePrivacy cookies or localStorage will not change the game either

Thanks!