Build an absolute url from an relative url and an absolute base url.

absurl.js

/**
 * Build an absolute url using an base url.
 * The provided base url has to be a valid absolute url. It will not be validated!
 * If no base url is given the document location is used.
 * Schemes that behave other than http might not work.
 * It tries to support file:-urls, but might fail in some cases.
 * email:-urls aren't supported at all (don't make sense anyway).
 *
 * NOTE: Internet Explorer does not support the baseURI property, but it does
 *       support the base tag, which results in wrong url. You might want to
 *       fix this by searching for a base tag and using its href attribute.
 *       If you use jQuery you might want to replace the first line of the
 *       function with this:
 *       if (!base) base = document.baseURI || $("html > head > base").last().attr("href") || document.location.href;
 */
function absurl (url, base) {
	if (!base) base = document.baseURI || document.location.href;
	if (!url) {
		return base;
	}
	else if (/^[a-z][-+\.a-z0-9]*:/i.test(url)) {
		// The scheme actually could contain any kind of alphanumerical unicode
		// character, but JavaScript regular expressions don't support unicode
		// character classes. Maybe /^[^:]+:/ or even /^.*:/ would be sufficient?
		return url;
	}
	else if (url.slice(0,2) === '//') {
		return /^[^:]+:/.exec(base)[0]+url;
	}
	
	var ch = url.charAt(0);
	if (ch === '/') {
		if (/^file:/i.test(base)) {
			// file scheme has no hostname
			return 'file://'+url;
		}
		else {
			return /^[^:]+:\/*[^\/]+/i.exec(base)[0]+url;
		}
	}
	else if (ch === '#') {
		// assume "#" only occures at the end indicating the fragment
		return base.replace(/#.*$/,'')+url;
	}
	else if (ch === '?') {
		// assume "?" and "#" only occure at the end indicating the query
		// and the fragment
		return base.replace(/[\?#].*$/,'')+url;
	}
	else {
		var base, path;
		if (/^file:/i.test(base)) {
			base = "file://";
			path = base.replace(/^file:\/{0,2}/i,'');
		}
		else {
			var match = /^([^:]+:\/*[^\/]+)(\/.*?)?(\?.*?)?(#.*)?$/.exec(base);
			base = match[1];
			path = match[2]||"/";
		}
	
		path = path.split("/");
		path.pop();
		if (path.length === 0) {
			// Ensure leading "/". Of course this is only valid on
			// unix like filesystems. More magic would be needed to
			// support other filesystems.
			path.push("");
		}
		path.push(url);
		return base+path.join("/");
	}
}