Using `"allowJs": true` and `"module": "commonjs"` to transform `.mjs` files should emit `.cjs` files

Bug Report

🔎 Search Terms

CommonJS, allowJS, ESM, CJS, file extensions

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about output "file extensions for CommonJS modules".

⏯ Playground Link

Sandbox link with relevant code (I couldn't use the playground, since this involves using an .mjs file).

💻 Code

// package.json
{
  "name": "tsc-mjs-extension-bug",
  "version": "0.1.0",
  "type": "commonjs",
  "main": "dist/javascript.mjs",
  "scripts": {
    "build": "tsc",
    "test": "node ."
  },
  "dependencies": {
    "typescript": "5.1.3"
  }
}

// tsconfig.json
{
  "compilerOptions": {
    "module": "commonjs",
    "moduleResolution": "nodenext",
    "allowJs": true,
    "outDir": "dist"
  }
  "include": ["src"]
}

// src/javascript.mjs
export const where = "javascript";
console.log(`Hello ${where}`);

// src/typescript.ts
export const where = "typescript";
console.log(`Hello ${where}`);

🙁 Actual behavior

The javascript.mjs is transformed to CommonJS (as expected), but the file extension of the emitted file is still .mjs.
This breaks the package, since .mjs is supposed to be use exclusively for JavaScript using ESM and the emitted file now uses CommonJS.

🙂 Expected behavior

I would expect tsc to transform the src/javascript.mjs to CommonJS and either:

1. emit it as dist/javascript.cjs or alternatively
2. take into account the "type": "commonjs" in the package.json and emit the file as .js as it does with the src/typescript.ts file.

Reproduisez le problème avec package.json, tsconfig.json, src/javascript.mjs et src/typescript.ts du rapport, puis exécutez les scripts de build et de test. Comparez les extensions et les formats de module générés dans dist ; le travail est considéré comme terminé lorsque l’entrée .mjs transformée en CommonJS est générée avec une extension valide pour ce format, avec une couverture du comportement choisi.