Occasionally it becomes useful to get the absolute form path of a URL from a relative one. You might be dynamically changing links on a page, or loading scripts from the same location as scripts that have already been loaded.

Computing an absolute URL by hand is problematic: you need to deal with any <base> elements in the document and properly implement the relative path canonicalization that browsers already do.

It turns out that on standards-compliant browsers based on Gecko, WebKit or Opera, the href property of anchor elements is magical. If you assign a URL fragment to it, it comes back as a fully-qualified URL when you read it back.

Ok, that’s great, but what about IE?  Well, it turns out that IE will fully-qualify URLs returned by the href property, but only if that anchor tag was created by the parser. Using createElement(‘a’) and setting href won’t trigger this code path.  There’s a trick we can use to work around this limitation, however.  You can force IE to use the parser to create elements by assigning innerHTML of another element.  This runs the element creation through the magic construction path that correctly sets up the attribute/property mapping.

Here’s a snippet of a function that will correctly canonicalize any URL you pass to it:

<script>
function canonicalize(url) {
    var div = document.createElement('div');
    div.innerHTML = "<a></a>";
    div.firstChild.href = url; // Ensures that the href is properly escaped
    div.innerHTML = div.innerHTML; // Run the current innerHTML back through the parser
    return div.firstChild.href;
}
</script>

You can use this script for other interesting purposes, like determining the base path for the current page (returns “/” for “/foo” and “/foo/” for “/foo/”). It gets the relative URL for the path “_#”, which removes any anchors, query strings or filenames from the current URL.

<script>
function getBasePath() {
    return canonicalize("_#").slice(0,-2);
}
</script>

The above code was tested on Safari, Chrome, Firefox 3.5 and IE6.