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Problem/Motivation

I'm interested in altering/overriding the little bit of HTML markup that's output by core, in my case Drupal.theme.mediaEmbedEditButton (from core/modules/media/js/media_embed_ckeditor.theme.js), but without hacking core, of course. It seems I could do this in a custom theme by adding:

libraries-extend:
  media/media_embed_ckeditor_theme:
    - my_theme/my_library

(plus the info for my_library in my_theme.libraries.yml, and the JS file in my theme with the function that would return the new markup).

However, since this shows up in places like /node/add/page that only uses the admin theme (we're using Seven), such additions to my_theme wouldn't apply. While it seems like I could make a bare-bones my_admin_theme, pulling in Seven as base, I'd like to keep this override near other related customizations in a custom module (I also ran into other cans of worms when I tried this as a proof of concept).

What (else) I've tried

I tried putting in libraries-extend: from above in my_module.info.yml (plus the addition to my_module.libraries.yml and the new JS, of course), but this didn't seem to work (kinda not surprising, since the docs I'm finding only seem to reference libraries-extend in the same breath as themes). I also tried using hook_field_widget_form_alter() in my_module.module to attach my override library via $element['#attached']['library'][] but didn't have luck there either.

Am I stuck with with making a custom admin theme if I want to use libraries-extend? (I'm guessing this is also a problem for libraries-override.) Or am I missing some hook or something I could instead add to my custom module to accomplish overriding the output made by Drupal.theme.mediaEmbedEditButton?

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    It does look like extending is for themes only, but even so - it seems a shame to override and maintain your own version of the whole library for this (if a new method gets added in the core version you'll have to add it in your own version and so on). I'd be tempted to just monkey patch that one function - declare your own library with a JS file containing (a more complete version of) Drupal.theme.mediaEmbedEditButton = function() { return 'html'; }. Make sure your library has a dependency on the core library, so its JS is included first, and the browser should do the rest
    – Clive
    May 8 '20 at 19:18
  • My hope was indeed to have to maintain just a JS file basically containing little more than Drupal.theme.mediaEmbedEditButton = function() { return 'custom button html'; } (plus the necessary library-extend and custom library stuff to have it picked up). I definitely don't want to override and maintain a whole core library. Sorry if that wasn't clear from my long-winded text, haha. The small JS worked when I made a custom admin theme (other unrelated issues notwithstanding). Just thought it was odd to only be supported for themes, not modules. Maybe I should file a feature request. May 8 '20 at 20:11
  • @MaxStarkenburg this is a great question, well-researched and documented. I think the answer is clear, though: you will need to create a custom admin theme to avoid the chore of overriding and maintaining a whole core library. Sep 25 at 5:53
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It's not possible, read this: https://www.oreilly.com/library/view/drupal-8-development/9781788290401/080b0131-1b64-4900-8535-55c82dba4200.xhtml

Manipulating libraries from hooks

Modules have the ability to provide dynamic library definitions and alter libraries. A module can use the hook_library_info() hook to provide a library definition. This is not the recommended way to define a library, but it is provided for edge use cases.

Modules do not have the ability to use libraries-override or libraries-extend, and need to rely on the hook_library_info_alter() hook. You can check out this hook in core/lib/Drupal/Core/Render/theme.api.php or at https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!theme.api.php/function/hook_library_info_alter/8.

source: Drupal 8 Development Cookbook - Second Edition by Matt Glaman

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