Convert a Relative Path to Absolute Path with jQuery and ASP.NET AJAX

I was messing around with relative paths to files (e.g., “../images/error.gif”) and needed to convert them to absolute paths (e.g., “http://server/images/error.gif”) on the client but couldn’t figure out how. Then I saw this nifty trick to HTML encode things using jQuery and it gave me an idea.

String.toAbsolutePath = function(relativePath) {
  /// <summary>
  /// Converts a relative file path into an absolute file path.
  /// </summary>
  /// <param name="relativePath" type="String">
  /// The string with the relative path, like "../foo/bar.gif"
  /// </param>
  /// <returns type="String" />
  var path = $("<div style=\"background-image:url('" + relativePath + "');\"></div>").css("background-image");
  if (path.startsWith("url(")) {
    path = path.substring(4);
  if (path.endsWith(")")) {
    path = path.substring(0, path.length - 1);
  if (path.startsWith("\"")) {
    path = path.substring(1, path.length);
  if (path.endsWith("\"")) {
    path = path.substring(0, path.length - 1);
  return path;

Basically, I’m using the CSS style “background-image” and feeding in the relative path, then resolving it immediately. Turns out the browser converts that to an absolute path for you. At least, Firefox 3.5.3 and IE 7 do, which is what I was testing with at the time.

The path.startsWith and path.endsWith checks are because sometimes the URL comes back like: url(“http://server/images/error.gif”) …with the url(“”) wrapper, and sometimes it comes back like: http://server/images/error.gif …without the wrapper at all.

Note the String.startsWith and String.endsWith methods come from ASP.NET AJAX so if you wanted to do it in just jQuery, you’d have to regex your way out of it or do a little more brute force work.

Of course, in the end, I figured out a different way to do what I was doing so I didn’t actually need to convert the path at all, but I thought this was sort of neat so I’d post it for folks. I didn’t really test it in a bunch of browsers or anything, so YMMV. “Works on My Machine.”