Google Tag Manager gtm.js 1st party snippet with www.googletagmanager.com fallback

gtm-snippet-with-fallback.html

<!-- Google Tag Manager -->
<script>
(function (w, d, s, l, i) {
	w[l] = w[l] || [];
	w[l].push({ "gtm.start": new Date().getTime(), event: "gtm.js" });
	var f = d.getElementsByTagName(s)[0],
		j = d.createElement(s),
		dl = l != "dataLayer" ? "&l=" + l : "";
	j.async = true;
	j.onerror = function () {
		var j2 = d.createElement(s);
		j2.async = true;
		j2.src = "https://www.googletagmanager.com/gtm.js?id=" + i + dl;
		f.parentNode.removeChild(j);
		f.parentNode.insertBefore(j2, f);
	};
	j.src = "https://data.wolfenthal.com/gtm.js?id=" + i + dl;
	f.parentNode.insertBefore(j, f);
})(window, document, "script", "dataLayer", "GTM-NLP8JVR");
</script>
<!-- End Google Tag Manager -->