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.

レポートにある package.json、tsconfig.json、src/javascript.mjs、src/typescript.ts を使って問題を再現し、その後 build スクリプトと test スクリプトを実行します。dist に出力された拡張子とモジュール形式を比較します。完了とは、CommonJS に変換された .mjs 入力が、その形式で有効な拡張子で出力され、選択した動作がテストでカバーされていることを意味します。