Generate ESM functions for Netlify

[eduardoboucas]

Netlify currently transpiles serverless functions with ESM syntax down to CJS, which is causing problems such as #6462.

We've started to address that in netlify/zip-it-and-ship-it#1198, which emits ESM files for functions with a .mjs extension. Because this is technically a breaking change, we're rolling it out gradually, but we've created a configuration property that allows frameworks like SvelteKit to opt-in to this behaviour straight away. This is done with a per-function configuration file introduced in netlify/zip-it-and-ship-it#1030.

With that in place, this PR has the following changes:

• The conditional logic that decided whether to create CJS or ESM functions was removed, and it now generates ESM only
• The generated function files use the .mjs extension
• There's an additional file generated for each function, containing the configuration property mentioned above

We're in the final stages of rolling out this functionality to all customers, but wanted to get the PR in front of you sooner rather than later to gather any feedback and improve the experience for SvelteKit users as soon as possible.

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[benmccann]

Thank you! Thank you! THANK YOU!

The generated function files use the .mjs extension

Is it necessary to use the .mjs extension if a user has "type": "module" in their package.json? (which all SvelteKit users will have)

We're in the final stages of rolling out this functionality to all customers, but wanted to get the PR in front of you sooner rather than later to gather any feedback and improve the experience for SvelteKit users as soon as possible.

To clarify, would it be okay if we merge this PR and roll it out to our users now or do we need to wait for something on your side still?

[eduardoboucas]

Is it necessary to use the .mjs extension if a user has "type": "module" in their package.json? (which all SvelteKit users will have)

For now, yes. We'll gradually roll out ESM support for .js files, but using .mjs allows you to opt-in to this behaviour straight away.

To clarify, would it be okay if we merge this PR and roll it out to our users now or do we need to wait for something on your side still?

We need to update our CLI with this change, which we'll do today. I'll keep this thread updated and let you know once it's good to merge.

[ascorbic]

Does this work properly if somebody has manually set their bundler to esbuild?

[eduardoboucas]

Does this work properly if somebody has manually set their bundler to esbuild?

Yes, esbuild will respect these settings.

[eduardoboucas]

We've released CLI v11.7.1 with support for these changes. I still want to do a couple of end-to-end tests, but I'm not sure I'll be able to do that today. It's on the top of my list for Monday, though!

[eduardoboucas]

This is all good to go from our side. Let me know if there's anything else that I need to do to get this merged and released.

Thanks! 🙌🏻

[benmccann]

Awesome!! Thank you so much!