feat: Supports ES SSR build based on module type

[rschristian]

Summary

Closes #2152

When "type": "module" is set for a package, vite build --ssr will not build to CJS, but ESM

I'm not sure if it's worth adding a test for this? It would basically just be a test checking the the ability to read the package file and Rollup's ability to output ES. Let me know if I should though, I certainly can.

[koumaza]

+1
LGTM. For project using Svelte-kit, not being built as ES is fatal...

[GrygrFlzr]

Could you rebase this against the main branch? There's been some shuffling around of some code and this section is now in the buildOuputOptions function.

vite/packages/vite/src/node/build.ts

Lines 399 to 403 in 1eb648e

	const buildOuputOptions = (output: OutputOptions = {}): OutputOptions => {
	return {
	dir: outDir,
	format: ssr ? 'cjs' : 'es',
	exports: ssr ? 'named' : 'auto',

Yes, with the typo of "Output" -> "Ouput", oops. Would be great if you can also fix the typo. 😬

[rschristian]

No worries! Rebased and corrected that typo

[patak-dev]

@rschristian ssr externals are imported using require at this point:

vite/packages/vite/src/node/ssr/ssrExternal.ts

Line 11 in eb66b43

* TODO right now externals are imported using require(), we probably need to

Could you check if this needs reworking as part of this PR?

[rschristian]

I'll take a peek, yeah. Worked when I opened this PR, but that was a couple months ago now.

[rschristian]

@patak-js Took me a bit to finagle the CJS SSR example into shape, but ESM server output is generated and everything seems to be correct / as I'd expect it with this PR.

[GrygrFlzr]

We discussed this PR with Evan:

• This change needs a playground test that uses "type": "module", we could change one of the ssr playgrounds to match.
• This needs to be configurable, as there are situations in which you may want to build an ESM project to a CJS output (e.g. serverless functions currently require a CJS entrypoint).

  ssr.format?: 'cjs' | 'es'

  This will override the SSR output. When the configuration option is not present, we can fall back to this PR's default of assuming "type": "module" = ESM.
• We'll need to expose an SSR flag for the Vite config like so:

  export default ({ command, mode, ssr }) => {

  Where ssr is a boolean that is true in SSR mode. This also needs a test.

[rschristian]

Will try to get that done next block of time I can find.

[rschristian]

* This needs to be configurable, as there are situations in which you may want to build an ESM project to a CJS output (e.g. serverless functions currently require a CJS entrypoint).
  ```ts
  ssr.format?: 'cjs' | 'es'
  ```

@GrygrFlzr Are you looking for a ssr.format or ssrFormat? I'm assuming that's a typo and you meant the latter?

* We'll need to expose an SSR flag for the Vite config like so:
  ```js
  export default ({ command, mode, ssr }) => {
  ```
  
  Where `ssr` is a boolean that is true in SSR mode. This also needs a test.

Also not super sure how this relates to this PR in anyway? Seems like something that should be separate?

[rschristian]

I added a new playground that uses "type": "module" as the test case here is super-simple and we don't really need to get involved with SSR in any way. Even having this behind SSR is a bit silly, as it's just post-build processing in native ESM, but I digress.

That being said, I couldn't figure out your test suite at all. Frankly I have no clue what's going on it in, way too much magic for my Sunday night brain. Essentially just need to make sure that vite build --ssr src/<something>.js on a module with "type": "module" outputs ESM and vite build --ssr src/<something>.js --ssrFormat cjs with "type": "module" outputs CJS.

Regardless, I'm on vacation for the next 2-ish weeks and won't be touching this for the duration. I'm not a user of Vite anyhow, and have 0 familiarity with the code base, so feel free to take this PR over and push to it or what not. You'll know better what you're looking for here.