feat!: move to ESM-only (CJS generation format also dropped)

[userquin]

Description

This PR includes the following changes:

• move the repo to ESM-only: type now is module
• removed format from SSG options, CJS output format removed
• use resolve instead join on the build module to avoid problems with Windows file system
• resolve server entry point in build module using pathToFileURL(resolve(...)).href removing the prefix and win32 check
• removed client argument from createApp function (client, spa and build modules)
• renamed eslint.config.mjs to eslint.config.js
• changed all package exports to use .mjs without types (typesVersions still there to support Node10)
• updated unbuild config declaration to node16 and removed rollup.emitCJS
• docs: update readme file including ESM-only from v27.0.0 (CJS generation format also dropped). at top and new tree-shaking section
• deprecate isClient

Will require changes from this PR #447 to test this PR in local examples...

Linked Issues

Additional context

From this comment #442 (comment)

[Copilot]

Pull Request Overview

This PR transitions the project to ESM-only by removing support for CJS generation, updating file system path handling, and revising function signatures to simplify usage. Key changes include:

• Removal of the "format" option and related CJS logic in the configuration and build process.
• Refactoring of path functions to use resolve and pathToFileURL for consistency and Windows compatibility.
• Updates to client-side entry functions and documentation to reflect the ESM-only approach.

Reviewed Changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/types.ts	Removed the "format" option from ViteSSGOptions to enforce ESM-only mode.
src/node/build.ts	Updated path handling and removed CJS-specific logic; adjusted createApp signature.
src/client/single-page.ts	Simplified createApp by removing the unused client parameter.
src/client/index.ts	Simplified createApp function signature by removing the client parameter.
build.config.ts	Updated build configuration for ESM-only mode and removed emitCJS option.
README.md	Updated docs to announce ESM-only support and added a tree-shaking guide.

Files not reviewed (1)

• package.json: Language not supported

[userquin]

don't merge yet, I'm going to update pwa plugin to 1.0.0, I'm having some issues with workbox-* deps resolving to lower 7.0.0 and should use 7.3.0 once pwa plugin updated to 1.0.0

done

[userquin]

We'll need to add some test to test Rollup tree-shaking using isClient in some example.

@kevinmarrec iirc you want to add more tests here, we should check previous comment before releasing this 🤞

[userquin]

NOTE: @kevinmarrec the isClient will not be removed, what I want to test is the code behind !ctx.isClient being tree-shaked from client asset, and should work using (ctx) => { if (!ctx.isClient) console.log('SERVER') }

ofc, if isClient can also be removed it would be nice => in my previous example, the callback should be also removed since there is no code to run in the client, I guess Rollup cannot three-shake that (maybe adding #__NO_SIDE_EFFECTS__ annotation to the hook will work, we need to check the side effects 🤔 => rollup/rollup#5024 )

[kevinmarrec]

@userquin Please check this REPL and play with constants.js (simulated import.meta.env.SSR)

I think we should drop isClient and enforce import.meta.env.SSR in userland which is documented as the way to tree shake things anyway when dealing with client vs server code branches.

[userquin]

so, we just need to remove the entry from the readme here, thx

[kevinmarrec]

Tested this PR against my project, went smoothly, and won additional 0.01 Kb on client bundle size 🙈 thanks to

removed client argument from createApp function (client, spa and build modules)

[userquin]

We should deprecate the isClient from the types.

[kevinmarrec]

We should deprecate the isClient from the types.

Makes sense, isClient deprecation annotation + instruction "Use !import.meta.env.SSR instead"

[userquin]

We should deprecate the isClient from the types.

Makes sense, isClient deprecation annotation + instruction "Use !import.meta.env.SSR instead"

We should add also a @see <href> with the link to the section in the readme

[kevinmarrec]

LGTM