Use ESM for SSR builds

[benmccann]

Clear and concise description of the problem

Vite currently builds SSR code as CJS.

I think that is controlled here:

vite/packages/vite/rollup.config.js

Line 72 in 274c10e

format: 'cjs',

Suggested solution

ESM is the future, so it would be nice to make it the default. SvelteKit, Astro, Nuxt, vite-plugin-ssr, Hydrogen, and Vitedge are all using ESM already, so it's possible we could stop supporting CJS altogether

Alternative

We could keep CJS around as an option for 3.0 and then drop it in 4.0. There are only a couple known issues and they're both being worked on. It's unclear if they'd be blockers

• Netlify has two deploy modes: zip-it-and-ship-it (ZISI) and esbuild. The ZISI method doesn't support ESM yet (Use pure ES modules netlify/zip-it-and-ship-it#749)
• Hydrogen is looking to drop CJS support ([HackDays] CLI generate components + sections + metaobjects sync Shopify/hydrogen#1155) but has run into an esbuild issue (How to fix "Dynamic require of "os" is not supported" evanw/esbuild#1921). Hopefully the PR for it will be reviewed soon (Rewrite require() calls of Node built-ins to import statements when emitting ESM for Node evanw/esbuild#2067)

Additional context

This was originally suggested here: #8033 (comment)

Validations

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[bluwy]

Maybe #6812 would cover this too, but the PR is stale.

[aleclarson]

I don't really see the point in dropping CJS support. Why not let people do what they want?

[patak-dev]

I don't really see the point in dropping CJS support. Why not let people do what they want?

We haven't decided to drop support for CJS in v3 yet. I think it isn't going to be possible at this point. Maybe there is an intermediate option where we keep it as deprecated with a note of intent that is going to be removed in 6 months or so, or it will be kept but not maintained. Only as a way to give people an easier migration path to Vite v3.
Regarding why we may want to drop support for CJS SSR builds, they currently require the esbuild scanner and the previous externalization logic heuristics. We have cleaned this for the ESM builds, but IIUC we can't do the same for CJS. I think a third option would be to also switch the CJS SSR build to externalization by default, and if users want to use it, they'll have to deal with noExternal themselves.