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.

Reproduziere das Problem mit package.json, tsconfig.json, src/javascript.mjs und src/typescript.ts aus dem Bericht und führe anschließend die Build- und Testskripte aus. Vergleiche die ausgegebenen Erweiterungen und Modulformate in dist; erledigt bedeutet, dass die in CommonJS umgewandelte .mjs-Eingabe mit einer für dieses Format gültigen Erweiterung ausgegeben wird und das gewählte Verhalten durch Tests abgedeckt ist.