passthrough-utm-tags.js

(function() {
    window.addEventListener("DOMContentLoaded", (event) => {
        const hostname = window.location.hostname;
        const params = new URLSearchParams(window.location.search);
        const utm_params = [];
        params.forEach(function(value, key) {
            if (key.startsWith('utm_')) {
                utm_params.push(key+'='+value)
            }
        })
        utm_search = utm_params.join('&');
        if (!!utm_search) {
            document.querySelectorAll('a[href]').forEach(function(ele, idx) {
                if (hostname && ele.href.indexOf(hostname) == -1) {
                    ele.href = ele.href + (ele.href.indexOf('?') === -1 ? '?' : '&') + utm_search;
                }
            });
        }
    });
})();