Feature Request: allow change file extension of generated files from `.ts`

Suggestion

🔍 Search Terms

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✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript/JavaScript code
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.

⭐ Suggestion

add something like

{
  "compilerOptions": {
    "module": "nodenext",
    "targetExtension": ".cjs",
}

📃 Motivating Example

• when targetExtension is .cjs , all .ts will emit as .cjs, but .mts still is emit as .mjs
• when targetExtension is .mjs , all .ts will emit as .mjs, but .cts still is emit as .cjs

💻 Use Cases

[azu]

I’ve met same issue when I building dual package.

e.g. https://github.com/azu/check-ends-with-period/tree/v2.0.0 (It is invalid example as dual package) TypeScript source code is insrc/*.ts and package.json has "type": "module" field. Also, this repository has two tsconfig files.

• tsconfig.json : generates esm to module/*.js from src/*.ts
• tsconfig.cjs.json: generates cjs to lib/*.js from src/*.ts

I’ve defined exports field as follows, but this package was treated as ESM because "type": "module" is defined. As a result, This package can not be requred from CJS without dynamic import. (Node.js treats *.js file as ESM by "type": "module")

  "exports": {
    ".": {
      "require": "./lib/check-ends-with-period.js",
      "import": "./module/check-ends-with-period.js"
    }
  },

I could not found just works solution without using bundler/post scripts.

---

If targetExtension option exists, I can resolve this issue by tsconfig file.

• tsconfig.json + "targetExtension": ".mjs",: generates esm to module/*.mjs from src/*.ts
• tsconfig.cjs.json + "targetExtension": ".cjs",: generates cjs to lib/*.cjs from src/*.ts

However, this option will need to rewrite import path of source code. It oppsite TypeScript’s design goal.

[Josh-Cena]

Major use-case is building dual-package with tsc without any postbuild script, I think.