in the {theme}.libraries.yml why we need to declare the jquery js or any other library as a depedency :

    assets/lib/bootstrap-4.0.0-dist/js/bootstrap.js: {}
    assets/script.js: {}

     - core/jquery

an not just include it for example as a simple js, before all other js that depends on it :

    assets/jquery.js: {} # load dependency library before the dependent libraries.
    assets/lib/bootstrap-4.0.0-dist/js/bootstrap.js: {}
    assets/script.js: {}

in the 2nd scenario the js file ex: jquery will be loaded before the other js files.

so what is the difference?

  • 4
    If every module adds his own jQuery file, instead of the jQuery core dependency, you would end up with a LOT of unnecessary duplicated JS libs massivly bloating your website.
    – Hudri
    Dec 17, 2020 at 18:48
  • 5
    because that pulls in that library, not just the jquery file, where it (a library) may have a few files it needs to function right.
    – Kevin
    Dec 17, 2020 at 19:16
  • 1
    It also ensures if the file path changes for any reason, it doesn't break your library.
    – Kevin
    Dec 17, 2020 at 19:26
  • @Kevin thank u I get the point we depend on a library that can consist of a set of files, and not just one file Dec 17, 2020 at 20:53

1 Answer 1


A library is a set of files (CSS or JavaScript) which could be local or loaded from another site. Declaring a dependency from another library has the vantage of not requiring to know:

  • Which files need to be loaded
  • Where those files are located
  • Which dependencies that library has

Using a library and declaring its dependencies also means that:

  • A file is loaded only once
  • There is only a copy of a file for all the modules
  • A module that depends from a library doesn't need to update the list of files (which are part of that library) when the library changes its files

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