Minimal Google Analytics script in modern javascript (677 bytes minified)

analytics.js

((document, location, navigator) => {
  const domain = location.hostname.split(".")
  const match = document.cookie.match(/(^|; ?)_ga=GA1\.\d\.(\d+\.\d+)(;|$)/)

  // use existing client id or generate one
  const cid = match ? match[2] : ~~(2147483648 * Math.random()) + "." + ~~(Date.now() / 1000)

  // set cookie at highest possible domain level
  for (let i = domain.length; i--;) {
    const cookie = `_ga=GA1.${domain.length - i}.${cid}`
    document.cookie = `${cookie}; max-age=63115200; domain=${domain.slice(i).join(".")}`
    if (document.cookie.split(/; ?/).includes(cookie)) break
  }

  // alternatively set the cookie at a specified domain
  // document.cookie = `_ga=GA1.3.${cid}; max-age=63115200; domain=subdomain.example.com`

  // you can also use a custom storage mechanism for the client ID if you do not need to be compatible with the _ga cookie.

  window.track = (type, ec, ea, el, ev) => {
    const data = {
      v: 1,
      tid: "UA-XXXX-Y", // replace with your tracking id
      aip: 1,
      cid,
      t: type,
      dr: document.referrer,
      dt: document.title,
      dl: location.href,
      ul: navigator.language.toLowerCase(),
      sr: `${screen.width}x${screen.height}`,
      vp: `${innerWidth}x${innerHeight}`
    }

    if (ec) data.ec = ec
    if (ea) data.ea = ea
    if (el) data.el = el
    if (ev) data.ev = ev

    navigator.sendBeacon("https://google-analytics.com/collect", new URLSearchParams(data))
  }

  track("pageview")
})(document, location, navigator)

analytics.min.js

// minified ES2015 (677 bytes)
((e,a,t)=>{for(var i,o,n=a.hostname.split("."),c=e.cookie.match(/(^|; ?)_ga=GA1\.\d\.(\d+\.\d+)(;|$)/),r=c?c[2]:~~(2e9*Math.random())+"."+~~(Date.now()/1e3),l=n.length;l--&&(o=`_ga=GA1.${n.length-l}.${r}`,e.cookie=`${o};max-age=63115200;domain=${n.slice(l).join(".")}`,!e.cookie.split(/; ?/).includes(o)););track=((o,n,c,l,d)=>{i={v:1,tid:"UA-XXXX-Y",aip:1,cid:r,t:o,dr:e.referrer,dt:e.title,dl:a.href,ul:t.language.toLowerCase(),sr:`${screen.width}x${screen.height}`,vp:`${innerWidth}x${innerHeight}`},n&&(i.ec=n),c&&(i.ea=c),l&&(i.el=l),d&&(i.ev=d),t.sendBeacon("https://google-analytics.com/collect",new URLSearchParams(i))}),track("pageview")})(document,location,navigator)

// minified ES5 (819 bytes)
!function(e,n,t){for(var o,a,i,c,r,d="https://google-analytics.com/collect",s=n.hostname.split("."),l=e.cookie.match(/(^|; ?)_ga=GA1\.\d\.(\d+\.\d+)(;|$)/),h=l?l[2]:~~(2e9*Math.random())+"."+~~(Date.now()/1e3),g=s.length;g--&&(r="_ga=GA1."+(s.length-g)+"."+h,e.cookie=r+";max-age=63115200;domain="+s.slice(g).join("."),-1==e.cookie.split(/; ?/).indexOf(r)););track=((r,s,l,g,p)=>{if(o={v:1,tid:"UA-XXXX-Y",aip:1,cid:h,t:r,dr:e.referrer,dt:e.title,dl:n.href,ul:t.language.toLowerCase(),sr:screen.width+"x"+screen.height,vp:innerWidth+"x"+innerHeight},s&&(o.ec=s),l&&(o.ea=l),g&&(o.el=g),p&&(o.ev=p),t.sendBeacon)t.sendBeacon(d,new URLSearchParams(o));else{for(c in a=[],i=new XMLHttpRequest,o)a.push(k+"="+encodeURIComponent(o[c]));i.open("POST",d),i.send(a.join("&"))}}),track("pageview")}(document,location,navigator);

This is missing path=/ in the cookie.