fix: use explicit .js extensions for ESM-compliant imports

[kimjoar]

Fixes #53

This change fixes import resolution errors that occur when consuming loro-mirror in strict ESM environments.

Problem

The package used directory imports like from "./schema" which TypeScript compiles as-is. This causes two distinct failures:

1. Runtime (Node.js ESM / Vitest): "Directory import is not supported resolving ES modules" - Node.js ESM requires explicit file extensions.

2. Compile time (TypeScript with moduleResolution: "NodeNext"): "Module 'loro-mirror' has no exported member 'RootSchemaType'" - TypeScript can't resolve ./schema to ./schema/index.d.ts in strict mode.

Consumers using lenient resolution modes (bundler, legacy node) are unaffected.

Solution

1. Changed tsconfig.json to use module: "NodeNext" and moduleResolution: "NodeNext"
2. Added explicit .js extensions to all relative imports

Sources

I find the tsconfig settings to be wildly confusing, but these seemed to indicate this was the right choice:

• How to Create an NPM Package by Matt Pocock (Total TypeScript)
• Is NodeNext right for libraries? by Andrew Branch (TypeScript team)

Consequences

• All relative imports must now include explicit .js extensions
• TypeScript will error at compile time if extensions are missing
• Consumers no longer need workarounds like Vitest's server.deps.inline
• Works with all TypeScript moduleResolution modes (bundler, node, NodeNext)

[Copilot]

Pull request overview

This PR fixes ESM import resolution issues by adding explicit .js extensions to all relative imports and updating TypeScript configuration to use NodeNext module resolution. The changes ensure compatibility with strict ESM environments (Node.js ESM, Vitest) and TypeScript's modern module resolution modes, addressing issue #53 where consumers experienced runtime and compile-time import resolution failures.

Key Changes

• Updated tsconfig.json to use "module": "NodeNext" and "moduleResolution": "NodeNext" for strict ESM compliance
• Added explicit .js extensions to all relative imports across source and test files (60+ import statements)
• Maintained consistent import patterns across the monorepo (core, react, and jotai packages)

Reviewed changes

Copilot reviewed 31 out of 32 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tsconfig.json	Updated TypeScript configuration to use NodeNext module resolution for ESM compliance
packages/core/src/index.ts	Added .js extensions to schema and core module imports
packages/core/src/core/index.ts	Added .js extension to mirror module import
packages/core/src/core/mirror.ts	Added .js extensions to loroEventApply, schema, utils, diff, and constants imports
packages/core/src/core/utils.ts	Added .js extensions to schema, mirror, and constants imports
packages/core/src/core/diff.ts	Added .js extensions to schema, mirror, utils, and constants imports
packages/core/src/core/loroEventApply.ts	Added .js extension to utils import
packages/core/src/core/loroEventApply.test.ts	Added .js extension to loroEventApply import
packages/core/src/schema/index.ts	Added .js extensions to types and validators imports
packages/core/src/schema/validators.ts	Added .js extensions to types and utils imports
packages/react/src/index.ts	Added .js extension to hooks import
packages/react/tests/readme.react.context.test.ts	Added .js extension to index import
packages/jotai/tests/readme.jotai.test.tsx	Added .js extension to index import
packages/jotai/tests/index.test.tsx	Added .js extension to index import
packages/core/tests/*.test.ts	Added .js extensions to all relative imports across 15 test files

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[kimjoar]

(Copilot PR review didn't impress compared to Codex 🫣)

Should have included this to begin with, didn't find a smooth way to get it into a test in the repo without adding more test setup.

Before:

~/dev/loro-mirror main
❯ pnpm build

> [email protected] build /Users/kimjoar/dev/loro-mirror
> pnpm -r build

Scope: 3 of 4 workspace projects
packages/core build$ tsc -p .
└─ Done in 1s
packages/react build$ tsc -p .
└─ Done in 786ms
packages/jotai build$ tsc -p .
└─ Done in 752ms

~/dev/loro-mirror main
❯ node --input-type=module -e "import './packages/core/dist/index.js'"
node:internal/modules/esm/resolve:262
    throw new ERR_UNSUPPORTED_DIR_IMPORT(path, basePath, String(resolved));
          ^

Error [ERR_UNSUPPORTED_DIR_IMPORT]: Directory import '/Users/kimjoar/dev/loro-mirror/packages/core/dist/schema' is not supported resolving ES modules imported from /Users/kimjoar/dev/loro-mirror/packages/core/dist/index.js
    at finalizeResolution (node:internal/modules/esm/resolve:262:11)
    at moduleResolve (node:internal/modules/esm/resolve:864:10)
    at defaultResolve (node:internal/modules/esm/resolve:990:11)
    at #cachedDefaultResolve (node:internal/modules/esm/loader:757:20)
    at ModuleLoader.resolve (node:internal/modules/esm/loader:734:38)
    at ModuleLoader.getModuleJobForImport (node:internal/modules/esm/loader:317:38)
    at #link (node:internal/modules/esm/module_job:208:49) {
  code: 'ERR_UNSUPPORTED_DIR_IMPORT',
  url: 'file:///Users/kimjoar/dev/loro-mirror/packages/core/dist/schema'
}

After:

~/dev/loro-mirror kim/fix-esm-imports
❯ pnpm build

> [email protected] build /Users/kimjoar/dev/loro-mirror
> pnpm -r build

Scope: 3 of 4 workspace projects
packages/core build$ tsc -p .
└─ Done in 969ms
packages/react build$ tsc -p .
└─ Done in 788ms
packages/jotai build$ tsc -p .
└─ Done in 758ms

~/dev/loro-mirror kim/fix-esm-imports
❯ node --input-type=module -e "import './packages/core/dist/index.js'"

Succeeds.

[zxch3n]

Thanks!