Paths in Web Components are Relative to Root

I am creating a web component using native implementation, which in it's html template has links to images. However, those links only work if they are absolute, or relative to the main document, which means, that that component is not reusable, or portable. Also, it is very counterintuitive.

Currently, I add a data-url_prefix attribute to all elements that need to use images. Then, when creating a shadow root for my custom element, I replace a {{URL_PREFIX}} with the value of that argument.

My solution seems very bad. I would be very glad if you advised something better, thanks.


I found an interesting quote on the http://webcomponents.org/polyfills/html-imports/ page:

POLYFILL NOTES

In imported documents, href and src attributes in HTML, and url properties in CSS files, are relative to the location of the imported document, not the main document.

why would a polifill use different logic that the native implementation?


Web Components Ideally should encapsulate all their dependencies, but if a web component requires an image, it should know the absolute URL to that image, which does not allow the component to be simply moved around in file structure.

Say, for example I have the following structure:

  • index.html
  • css
    • main.css
  • js
    • main.js
  • web_components
    • cool_web_component
      • cool_web_component.html
      • icon.png

If I change it to the following:

  • index.html
  • css
    • main.css
  • js
    • main.js
  • cool_web_component
    • cool_web_component.html
    • icon.png

I would be required to change the pointer to icon.png somewhere in those files My question is how to avoid it, or solve it in elegant way. Also, why the actual native implementation is in conflict with the polyfills?

Answers:

Answer

The webcomponent specification defines that URLs are always relative to the main document. This, of course, breaks web component encapsulation, as you rightly concluded. In other words, the specification is buggy, and lots of people are complaining about it. That's why the polyfill doesn't follow the specification: it's fixing the problem.

The specification will change. However, since this is not trivial to fix, it could still take some time. Please see these links:

https://lists.w3.org/Archives/Public/public-webapps/2014OctDec/0013.html

https://www.w3.org/Bugs/Public/show_bug.cgi?id=20976#c8

For the moment, the solution is for your component to change the URL of its template images, from relative to absolute. You can get the templateBaseUrl as follows:

(function (window, document)
    {
    var proto = Object.create(HTMLElement.prototype);

    var template = document.currentScript.ownerDocument.querySelector("template");
    var templateBaseUrl = template.baseURI;

    proto.createdCallback = function ()
        {
        // Now find all images inside the template and change their src attribute, 
        // using templateBaseUrl. Also you must change CSS background-image: url(...);
        ...
        };

    document.registerElement('test-element', {prototype: proto});
    })(window, document);

Another way of fixing this, in case of small images like icons, is to embed the image data directly into the document, with data URIs. This also saves HTTP Requests (until we have Http/2). For example:

<img src="data:image/gif;base64,R0lGODlhEAAQAMQAAORHHOVSKudfOulrSOp3WOyDZu6QdvCchPGolfO0o/XBs/fNwfjZ0frl3/zy7////wAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACH5BAkAABAALAAAAAAQABAAAAVVICSOZGlCQAosJ6mu7fiyZeKqNKToQGDsM8hBADgUXoGAiqhSvp5QAnQKGIgUhwFUYLCVDFCrKUE1lBavAViFIDlTImbKC5Gm2hB0SlBCBMQiB0UjIQA7" />
Answer

This behavior seems to be specific to images.
For script and link tags, relative paths from imported document work as expected.

Also I noticed that this is not something specific to polyfills, even for native implementation(chrome) this issue(?) seems to be there.

Seems like only option here is to include a script in your imported html, which will convert these relative paths to their absolute counterparts.
To solve it elegantly you can avoid hardcoding urls in your script and generate it using url of your importing document. You can get that from document.currentScript.ownerDocument(or document._currentScript.ownerDocument in polyfilled scenario).

So to answer your second question, I don't see any difference in native and polyfilled implementation at least in terms of behavior of src and href attributes.
Quote you mentioned from http://webcomponents.org/polyfills/html-imports/ seems to be specific to href/src of script and link tags and they work as written.

Hope it helps.

Answer

Reacting on my own prev comment to MarcG answer - the need for the modules to be aware of their own path has become acknowledged and nowadays may be retrieved by the import.meta.url mean!

So it is much easier to write now some util to fetch component's HTML/CSS.

Furthermore, if CSS is NOT fetched, but a new style sheet link dynamically added to the document - this CSS plays very well with relative paths furthermore.

Having that, today, while waiting for some standard way to specify my component's base url, my ow approach is to provide a component with 2 resources:

  • main/single HTML is fetched via fetch API, turned into template, cached and then cloned into the component each time new instance of it attached to the DOM
  • main CSS is linked by the standard link element
  • names of those 2 are the same as the main JS file providing component's class - making it easier to turn this whole mess into a dozen-of-lines-util
  • additional CSS resources, if needed, added as a regular relative imports in the main CSS
  • images, if possible, added as url backgrounds using relative imports as well
  • no more than singe HTML resource obviously

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