Add the outputExtension compiler option

[@NullVoxPopuli's reduced-access machine account for AI usage (NullVoxPopuli-ai-agent)]

Fixes #64053

A new compiler option, outputExtension, tells TypeScript which file extension an external build gives every source file:

{
  "compilerOptions": {
    "declaration": true,
    "emitDeclarationOnly": true,
    "outputExtension": ".js"
  },
  "contentMappers": [
    { "package": "ember-content-mapper", "extensions": [".gts", ".gjs"] }
  ]
}

With this configuration, counter.gts emits counter.d.ts instead of counter.d.gts.ts. In emitted declarations, ./counter.gts becomes ./counter.js and ./format.ts becomes ./format.js.

Why

Declaration emit names a content-mapped file counter.d.gts.ts and keeps ./counter.gts in specifiers. That is the only safe choice when TypeScript knows nothing about the external build. When that build compiles counter.gts to counter.js (Ember, MDX), the published package needs counter.d.ts next to counter.js, and consumers without the mapper cannot resolve the .gts specifiers.

The same build compiles .ts files, so the option is a program-wide compiler option rather than a per-mapper setting. This answers the question in #64053 about how TypeScript can know what the build tool emits: the project declares it.

Behavior

• Accepted values are .js, .mjs, and .cjs. Declaration files are named to match: .d.ts, .d.mts, or .d.cts. This applies to .ts, .tsx, .js, .jsx, and content-mapped files.
• Declaration emit rewrites relative specifiers that resolved to a source file by its source extension. ./a.ts, ./b.tsx, and ./c.gts become ./a.js, ./b.js, and ./c.js. This covers imports, exports, import() types, and import x = require().
• Specifiers that already use the output extension, or no extension, are left as written. So are non-relative specifiers and unresolved paths.
• The option requires noEmit or emitDeclarationOnly. Otherwise tsc reports TS5069. The external build owns the JavaScript output, so tsc must not also emit it.
• Any other value reports TS6046 on the value in tsconfig.json.
• Without outputExtension, nothing changes.

The name collision from the issue thread (app.ts next to app.gts, both emitting app.d.ts) is reported by the existing TS5056 check.

Tests

• outputExtension: .js with a content mapper, imports between .ts and mapped files, import() types, .js and extensionless specifiers left alone, declaration maps.
• outputExtensionMjs: .mjs without a content mapper. .ts and .tsx files emit .d.mts.
• outputExtensionInvalidValue: TS6046 for .jsx.
• outputExtensionRequiresEmitDeclarationOnly: TS5069 without noEmit or emitDeclarationOnly.
• outputExtensionConflict: app.ts and app.y.z both emit app.d.ts.

The built compiler also ran on ember-content-mapper/examples/cli-library-v2 with the option added. The declarations/ directory then holds only .d.ts files, and index.d.ts imports ./components/counter.js and ./utils/format.js.

Disclosure

This patch was written by Claude Code (Claude Fable 5) from the NullVoxPopuli-ai-agent account. The operator is NullVoxPopuli, who filed #64053 and maintains ember-content-mapper. The PR stays a draft until NullVoxPopuli has reviewed it, and NullVoxPopuli will respond to review feedback.

NullVoxPopuli please review.

🤖 Generated with Claude Code

https://claude.ai/code/session_0188eg6bTrrUiWbj5FZaWH5Z

[@NullVoxPopuli's reduced-access machine account for AI usage (NullVoxPopuli-ai-agent)]

Reworked after operator feedback: outputExtension is now a compiler option that applies to the whole program, not a per-mapper field. The external build compiles .ts files too, so .ts declarations and their relative imports follow the same rule. The option now requires noEmit or emitDeclarationOnly. The PR description is updated to match.

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[NullVoxPopuli]

idk if it matters to restrict this much, but such restriction does help prevent misuse

could go either way tho