var xhr = new XMLHttpRequest();
xhr.open('GET', 'https://dotjs.local/' + window.location.host.replace(/^www\./, '') + '.js');

xhr.onreadystatechange = function() {
	if(xhr.readyState == 4) {
		if(xhr.status == 200) {
			var script = document.createElement('script');
			script.textContent = xhr.responseText;
			document.head.appendChild(script);
			return;
		} else if(xhr.status == 404) {
			var defaultXHR = new XMLHttpRequest();
			defaultXHR.open('GET', 'https://dotjs.local/default.js');
			defaultXHR.onreadystatechange = function() {
				if (defaultXHR.readyState == 4 && defaultXHR.status == 200){
					var script = document.createElement('script');
					script.textContent = defaultXHR.responseText;
					document.head.appendChild(script);
					return;
				}
			}
			defaultXHR.send();
		}
	}
};

xhr.send();