Problems with __esModule marker in JS/TS mixed node16 environemnt about typescript

[fatcerberus]

If your .cts files contain text like

import syntheticDefaultImport from "./cjs-to-cjs-js.cjs";
import * as starImport from "./cjs-to-cjs-js.cjs";

instead of require() calls, then the resulting .cjs file absolutely is a transpiled ES module.

[meros]

I fully understand that, but the problem is not there. It's in the cjs-to-cjs-js.cjs file:

exports.message = "This is a CommonJS module transpiled to CommonJS (expect no marker).";

This is transpiled to:

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.message = "This is a CommonJS module transpiled to CommonJS (expect no marker).";

Note the incorrect Object.defineProperty(exports, "__esModule", { value: true }); line. This causes the TypeScript syntheticDefault function to fail (as it should if this were really a transpiled ESM).

Here is the function that is generated by default import calls translated to require:

var __importDefault = (this && this.__importDefault) || function (mod) {
    return (mod && mod.__esModule) ? mod : { "default": mod };
};

The problem seems to be in shouldEmitUnderscoreUnderscoreESModule that only checks that the file is external and has no 'exportEquals', which is the TypeScript construct:

export = ...

Note that this construct doesn't work with real CommonJS code (no matter if it's in a TS source or JS source) and thus wrongly emits the marker:

exports = ...
module.exports = ...

A reasonable solution could be to extend the test to other variants of exports =, but I don't know enough about the code to understand how this can be done reliably. Another reasonable variant is to never emit this for js files that are indicated to be commonjs already (based on filename, there is a system for this in place)

[andrewbranch]

The definite bug is that the checking and the emit are misaligned. The checking assumes a “true“ CJS module while the emit produces a transpiled ESM-transpiled-to-CJS module. And I agree the input JS should be the source of truth here; i.e. it should emit as a true CJS module.