attribution.js

https://jennamolby.com/how-to-use-cookies-to-capture-url-parameters/

// Parse the URL
function getParameterByName(name) {
	name = name.replace(/[\[]/, "\\[").replace(/[\]]/, "\\]");
	var regex = new RegExp("[\\?&]" + name + "=([^&#]*)");
	var results = regex.exec(location.search);
	return results === null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
}

// Give the URL parameters variable names
var utm_source = getParameterByName('utm_source');
var utm_medium = getParameterByName('utm_medium');
var utm_campaign = getParameterByName('utm_campaign');

function getCookie(cname) {
    var name = cname + "=";
    var decodedCookie = decodeURIComponent(document.cookie);
    var ca = decodedCookie.split(';');
    for(var i = 0; i <ca.length; i++) {
        var c = ca[i];
        while (c.charAt(0) == ' ') {
            c = c.substring(1);
        }
        if (c.indexOf(name) == 0) {
            return c.substring(name.length, c.length);
        }
    }
    return "";
}

function setCookie(cname, cvalue) {
    const d = new Date();
    // On Safari, max duration for cookies is 7 days.
    d.setTime(d.getTime() + (30 * 24 * 60 * 60 * 1000)); // 30 = number of days
    const expires = `expires=${d.toUTCString()}`;
    // Using the new SameSite attribute: https://web.dev/samesite-cookie-recipes/
    document.cookie = `${cname}=${cvalue};${expires};path=/;SameSite=Strict`;
    // Fallback for legacy browsers: https://web.dev/samesite-cookie-recipes/
    document.cookie = `${cname}=${cvalue};${expires};path=/;Secure`;
}
 
// Set the cookies
if(utm_source != "" && getCookie("utm_source") == "") {
    setCookie('utm_source', utm_source);
}
if(utm_medium != "" && getCookie("utm_medium") == "") {
	setCookie('utm_medium', utm_medium);
}
if(utm_campaign != "" && getCookie("utm_campaign") == "") {
	setCookie('utm_campaign', utm_campaign);
}
if(document.referrer != "" && getCookie("referrer") ==  "") {
    setCookie('referrer', document.referrer);
}