Typescript [4.8.2] is adding invalid javascript for *.cjs files

Bug Report

My project is set to emit ESNext modules. I have a variety of TS files, but I also have a single .cjs file, which needs to remain CommonJS to try and require legacy packages.

My input looks like this:

module.exports = {
  tryRequire(modulePath) {
    return require(modulePath);
  },
};

I expect the output I get from TypeScript 4.7.4:

module.exports = {
  tryRequire(modulePath) {
    return require(modulePath);
  },
};

When I upgrade to TypeScript 4.8.2, it now emits an extra export {} at the end of the .cjs file:

module.exports = {
  tryRequire(modulePath) {
    return require(modulePath);
  },
};
export {}

This causes Node to fail parsing the .cjs file because export does not exist in the .cjs context.

🕗 Version & Regression Information

Worked in 4.7.4, broken in 4.8.2, also broken in current nightly (4.9.0-dev.20220905).

Repro here

https://github.com/dzearing/ts-repro-cjs

[dzearing]

I am unblocked here. I do still think it’s not ideal to be emitting invalid cjs but at least I have a workaround. 😃

[dzearing]

Yes! That fixed it. I changed to module: "nodenext" and added type: "module" to the package.json to achieve the expected output (ESM by default, CJS for cjs files.)