Create CJS build output, or expect adapters to bundle the ESM themselves?

From https://github.com/sveltejs/kit/pull/485#issuecomment-797519054:

I almost wonder if adapters should be responsible for bundling the outputted ESM (this would also solve the question of what needs to be considered external during build, so that adapters that target lambda can create a single CJS file.

Alternatively maybe Vite could output both ESM and CJS from build.

tl;dr most lambda platforms seem to expect CJS, even though Node itself runs ESM natively. The best outcome would probably be if those adapters had access to a single CJS file with no external dependencies (since the platform’s bundlers don’t always work as you’d hope), though the possibility of native modules in the dep graph makes this a bit of a challenge. So exporting CJS from Vite alongside the ESM is probably the easiest solution, even if it’s a bit wasteful.

We could eliminate the waste if the adapter was able to declare its requirements to build.

[Rich-Harris]

See also: #612

[nhe23]

There also seems to be an issue with the node adapter not being able to resolve some modules like firebase:

The same error occurs for packages like rxfire and rxjs. All packages where used in their latest version. I tried using several node versions (14.16.0 & 12.21.0) which did not help.