Polyfill does not run as expected

[benj-p]

I have a Rails (7.1.3) app with importmap-rails (2.0.1). We noticed that when visiting the website with the Random User-Agent (Switcher) Chrome extension we get these error messages in the console and importmap does not work.

I would have expected the polyfill to run but it doesn't seem to be the case here. This is what I have in the head of my page

    %script{ async: true, src: "https://ga.jspm.io/npm:es-module-shims@1.8.2/dist/es-module-shims.js", "data-turbo-track": "reload" }
    = javascript_importmap_tags
    = javascript_import_module_tag 'controllers'

Any guidance would be greatly appreciated 🙏

[guybedford]

Import maps are supported natively in modern Chrome - so the polyfill will not apply if you are only relying on a feature like import maps that is already supported.

Instead, this is a bug with the code against the native implementation of import maps - that import maps must be loaded before any modules execute on the page and cannot be loaded after a single module has been loaded. Try carefully inspecting your HTML to ensure all module loads come after the import map.

[benj-p]

Thanks for the prompt answer @guybedford.

The page has no error and works as expected when browsed without the Random User-Agent (Switcher) Chrome extension. When the extension in enabled though, I get those error messages. I have inspected my HTML however and in both cases, whether the extension is enabled or disabled, I can confirm that there are no module loads before the import map - which is odd given the error message I'm seeing.

But what I understand from your reply is that the polyfill will not apply in Chrome, so I guess this issue is not related to es-module-shims. If that's correct I will close this issue and create one issue with importmap-rails instead.

[guybedford]

@benj-p you may find that the root cause here is WICG/import-maps#248. Unfortunately it's a bit of a gotcha, and is best treated as an "extension issue" actually currently.

[benj-p]

Thanks, I had indeed found this issue and I came here to post an issue following your comment ;)