Getting an absolute URL from a variable string isn't as easy as you think. There's the URL constructor but it can act up if you don't provide the required arguments (which sometimes you can't).

getAbsoluteUrl.js

var getAbsoluteUrl = (function() {
	var a;

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

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

// Usage
getAbsoluteUrl('/something'); // http://domain.name/something