Uncaught TypeError: a is undefined

The reason is, that if attaching the library via photoswipe/photoswipe.init the photoswipe container (from templates/photoswipe-container.html.twig) is missing on the page.

That's a problem I have no solution for yet ...

As the current error message is not very clear, let's at least add a better error message for now.

Let's merge this improved error message for now, until we have a solution or decide to not fix this and use v5.

Workaround for now: Enable the global option at /admin/config/media/photoswipe

@anybody There should be a hook to load the libraries, as you told me :P

So I think as long as the JS library needs this boilerplate code, but it's not added in the library by JS (which we intentionally don't do), there's no solution here.

For Photoswipe v4 aka module version 3.x the only solution for such custom cases is to call
\Drupal::service('photoswipe.assets_manager')->attach($attachments);
in a hook or add the template code manually on the page before attaching photoswipe/photoswipe.init library.

We won't fix this here for 3.x anymore.

In 🐛 Image Paragraph lightbox zoom misses to load photoswipe library Fixed I'll try to call
\Drupal::service('photoswipe.assets_manager')->attach($attachments);
via hook.