The history of libraries, part 1

During the early development of Drupal 8, assets such as javascript and stylesheets had their own, different way of getting imported into themes. Importing a custom javascript file was done using the scripts property inside the .info.yml-file of the theme. To import stylesheets, the stylesheets property was used. This was more or less the way it was done in Drupal 7. Here is an example from a Drupal 7 file.


# Adding stylesheets
stylesheets[all][] = css/styles.css
stylesheets[print][] = css/print.css

# Adding javascript files
scripts[] = js/script.js


An alternative path was chosen. Javascript maintainer _nod created an issue on using AMD for JS architecture. The patch for the issue became so big that a separate issue was created. This issue handled the splitting up of the dependencies. This patch, Explicitly declare all JS dependencies, don't use drupal_add_js, where all javascript files are declared as libraries and added the relevant dependencies to the scripts, got committed about two weeks after the creation. The original issue [AMD architecture] however was posponed to a later Drupal release (Drupal 9.x).


During the early stages of Drupal 8, this was done using hook_library_info. Since this was one of the last remaining hooks in Drupal 8, it got replaced by a *.libraries.yml file). This means modules, themes and profiles can define their own *.libraries.yml file. The file should be in the root directory of the theme (or module).

# example.libraries.yml

  version: 1.x
    js/scripts.js: {}
      css/components/component.css: {}
    - core/drupal
    - core/jquery

Inside the .info.yml file from a module or theme, the library can be included using the libraries property.


  - example/base

The example theme now has a global dependency on the base library (from the example theme/module); example/base. Therefor it's included on every page (where this theme is loaded). The base library itself contains a single javascript file (scripts.js), a css file (categoried as a component) and has dependencies on the drupal and jquery library from core (read more on this in the Javascript chapter).

The history of libraries, part 2

Due to this change, the scripts tag from the .info.yml, was removed and replaced with the libraries property. This means scripts can only be included using the libraries property and a libraries.yml file. Stylesheets however (in themes) could still included using the old stylesheets property:


remove scripts[] from info files and replace it with library[] …

stylesheets[] can be kept as is, it's totally fair to add a single css file without dependencies. It's highly unlikely for scripts, they will need to depend on jquery, drupal.js 99% of the time.

It became clear this new way of adding assets had some major advantages. Wim Leers created a new issue stating Themes should use libraries, not individual stylesheets. Due to the advantages pointed out in this issue, the stylesheets property from the .info file got removed. Thanks to this, there is now a unified way for adding assets libraries.