Any errors in the browser console?

My apologies for not providing the error message.

The error message says, 'JSZip is not defined'. 

I used this library on a non-Elgg site and it works without the above error. So that's why I wonder that I might do something wrong with registering 3rd-party libraries via elgg_define_js( ).

Here is the library I want to register - https://gitbrent.github.io/PptxGenJS/docs/installation.html

Try this:

elgg_define_js('JSZip', [
      'src' => '/mod/my_plugin/vendors/JS/pptxgen.bundle.js',
      'exports' => 'JSZip',
]);

Thanks Nikolai for your suggestion.

I tried your recommended approach. Then I used elgg_require_js('JSZip') on the page that I want to use this library. Now it throws different error - "net::ERR_ABORTED 403 (Forbidden)   require.js:1961". 

Just tested on my localhost with this:

elgg_define_js('PptxGenJS', [
  'src' => elgg_get_simplecache_url('js/pptxgen.bundle.js'),
  'exports' => 'PptxGenJS',
]);

/mod/my_plugin/views/default/js/pptxgen.bundle.js

Uncaught SyntaxError: Invalid or unexpected token

I couldn't replicate the error you got above after following your approach. The error I got was 'JSZip is not defined'. Anyways, I am going to search for other Elgg approaches to register 3rd-party JS libraries rather than elgg_define_js( ).

Thanks again Nikolai! Always much appreciated your help.

I am going to search for other Elgg approaches to register 3rd-party JS libraries rather than elgg_define_js( ).

Yes, you can try elgg_register_external_file function.

Look at these examples.