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Ben Nadel at Scotch On The Rock (SOTR) 2010 (London) with: David Phipps
Ben Nadel at Scotch On The Rock (SOTR) 2010 (London) with: David Phipps ( @phipps73 )

Base Tag HREF Doesn't Affect Document Root-Relative URLs

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At work, one of the things that our platform does is generate an offline version of a prototype. The code for generating this offline prototype has to jump through a lot of hoops in order to adjust file-paths for various images, CSS, and JavaScript files. I briefly wondered if I might simply this process by including a <base href=""> tag in the offline template. However, after some experimentation, it seems that the base tag's href property does not affect "Root-Relative" URLs.

Run this demo in my JavaScript Demos project on GitHub.

View this code in my JavaScript Demos project on GitHub.

When you include a src or href attribute, you can use various formats of URL:

  • Absolute URL: These URLs are fully-qualified and start with a protocol (ex, https://) and include a domain and an optional resource.

  • Relative URL: These URLs typically start with ./ and are resource-only paths that are relative to the current document location.

  • Root-Relative URL: These URLs start with / and are resource-only paths that are relative to the root of the site.

What I wanted to see is if this last format - those root-relative URLs that start with / - might be "remapped" using a <base> tag. To test this, I created a JavaScript file that queries for itself in the DOM (Document Object Model) and then logs-out its own URL "version".

This tag expects to be loaded with a ?v= in its own query-string. As in:


It will split the string on ? and log out the last item:

// Since the document is going to block-and-wait for this Script tag to load, it means
// that the last <script> element on the page is currently THIS ONE. As such, let's grab
// all of the script tags and then take the one in the last index.
var nodes = document.querySelectorAll( "script" );
var thisNode = nodes[ nodes.length - 1 ];

// Log the "?v=" portion of the SRC attribute.
console.log( "Loaded:", thisNode.src.split( "?" )[ 1 ] );

I then put this JavaScript in an assets folder and attempted to load it using both a relative URL and a root-relative URL in conjunction with a base tag that points the base-HREF to the assets folder:

<!doctype html>
<html lang="en">
	<meta charset="utf-8" />

		Base Tag HREF Doesn't Affect Document Root-Relative URLs

		Our "loader.js" script is stored within the "/assets/" folder. Let's see if we can
		load this file using different forms of SRC URLs in conjunction with a BASE tag
		HREF that places the current page context inside the assets folder. We're going to
		try using both a RELATIVE and a DOCUMENT ROOT-RELATIVE path.
	<base href="./assets/" />
	<script src="./loader.js?v=relative"></script>
	<script src="/loader.js?v=root-relative"></script>


What I was hoping for is that both of the src attributes would resolve to:


However, when we run this in the browser, we get an error on the root-relative script load:

Script with relative URL loads relative to base HREF; however, script with rool-relative URL fails to load, relative to site domain.

As you can see, the ?v=relative script tag loaded fine - its src attribute was affected by the base tag's href property. The root-relative script, with ?v=root-relative, failed load. Its src attribute still resolved relative to the domain, and was unaffected by the base tag.

Oh well! Nothing ventured, nothing gained. I was hoping that I might use the base tag's href property to "rewrite" some incorrect root-relative paths. But, it looks like that's not going to be an option.

Want to use code from this post? Check out the license.

Reader Comments



Oh, interesting. I've never heard that term before. That said, when I tried to Google for "root-relative", it's not like a ton of matches came up. It was hard to find the right language.

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