Edit in JSFiddle

var getAbsoluteUrl = (function() {
	var a;

	return function(url) {
		if(!a) a = document.createElement('a');
		a.href = url;

		return a.href;
	};
})();


// Test
console.log(getAbsoluteUrl("/some-relative-url"));
<html>
<body>
Check the console to see the output of getAbsoluteUrl("/some-relative-url")
</body>
</html>
body {
  font-family: arial;
}