Gets and sets a unique user id

getUUID.js

const getUUID = () => {
  const COOKIE_NAME = "gbuuid";
  const COOKIE_DAYS = 400; // 400 days is the max cookie duration for chrome

  // use the browsers crypto.randomUUID if set
  const genUUID = () => {
    if(window?.crypto?.randomUUID) return window.crypto.randomUUID();
    return ([1e7]+-1e3+-4e3+-8e3+-1e11).replace(/[018]/g, c =>
      (c ^ crypto.getRandomValues(new Uint8Array(1))[0] & 15 >> c / 4).toString(16)
    );
  }
  const getCookie = (name) => {
    let value = `; ${document.cookie}`;
    let parts = value.split(`; ${name}=`);
    if (parts.length === 2) return parts.pop().split(';').shift();
  }
  const setCookie = (name, value) => {
    var d = new Date();
    d.setTime(d.getTime() + 24*60*60*1000*COOKIE_DAYS);
    document.cookie = name + "=" + value + ";path=/;expires=" + d.toGMTString();
  }

  // get the existing UUID from cookie if set, otherwise create one and store it in the cookie
  if(getCookie(COOKIE_NAME)) return getCookie(COOKIE_NAME);
  
  const uuid = genUUID();
  setCookie(COOKIE_NAME, uuid);
  return uuid;
}

// minified version:
const getUUID=()=>{let $="gbuuid",e=()=>window.crypto.randomUUID?window.crypto.randomUUID():"10000000-1000-4000-8000-100000000000".replace(/[018]/g,$=>($^crypto.getRandomValues(new Uint8Array(1))[0]&15>>$/4).toString(16)),t=$=>{let e=`; ${document.cookie}`.split(`; ${$}=`);if(2===e.length)return e.pop().split(";").shift()},r=($,e)=>{var t=new Date;t.setTime(t.getTime()+3456e7),document.cookie=$+"="+e+";path=/;expires="+t.toGMTString()};if(t($))return t($);let i=e();return r($,i),i};

// Basic usage:
console.log(getUUID());

// GrowthBook usage with GA4:

<script>
  (function() {
    // Wait for the SDK to load before starting GrowthBook
    if (window.growthbook) {
      startGrowthbook();
    } else {
      document.querySelector("#growthbook-sdk").addEventListener("load", startGrowthbook);
    }

    function startGrowthbook() {
      if (!window.growthbook) return;
      const getUUID=()=>{let $="gbuuid",e=()=>window.crypto.randomUUID?window.crypto.randomUUID():"10000000-1000-4000-8000-100000000000".replace(/[018]/g,$=>($^crypto.getRandomValues(new Uint8Array(1))[0]&15>>$/4).toString(16)),t=$=>{let e=`; ${document.cookie}`.split(`; ${$}=`);if(2===e.length)return e.pop().split(";").shift()},r=($,e)=>{var t=new Date;t.setTime(t.getTime()+3456e7),document.cookie=$+"="+e+";path=/;expires="+t.toGMTString()};if(t($))return t($);let i=e();return r($,i),i};

      let gbuuid = getUUID();
      let gb = new growthbook.GrowthBook({
        apiHost: "https://cdn.growthbook.io",
        clientKey: "sdk-abcd1234",
        // TODO: Add decryptionKey if using encryption
        attributes: {
          id: gbuuid
        },
        trackingCallback: function(experiment, result) {
          // track with GA4, for example:
          gtag("event", "experiment_viewed", {
            event_category: "experiment",
            experiment_id: experiment.key,
            variation_id: result.variationId,
            gb_user_id: gbuuid,
          });
        }
      });

      // TODO: Instrument DOM with AB test logic
      gb.loadFeatures().then(function() {
        // if you want to do anything after GB loads
      });
    }
  })();
</script>