TypeScript module not follow specification in some cases when compilerOptions.module = commonjs

TypeScript Version: 3.9.7

When, in a index file, it is exported the content of another file, and tsconfig compilerOptions.module = commonJs, tsc transpiles it with read-only accessors, which is not complying with the behavior described in the official documentation

Search Terms: exports accessor, export readonly, export accessor readonly, export getter, compilerOptions module

Code tsconfig file is configured for commonJs module output

{
  "compilerOptions": {
    "outDir": "dist",
    "rootDir": "src",
    "target": "es2018",
    "module": "commonjs",
    "strict": true
  }
}

File my-test.ts

export function myTest() {
  return 1;
}

File index.ts

export * from "./my-test";

export function myTest2() {
  return 3;
}

Supposing it is transpiled to a dist folder, you can have a javascript file with this code:

const test = require('./dist');
test.myTest = function () {
  return 2;
}

console.log(test.myTest());

Alternative:

import test = require("./index");
Object.defineProperty(test, "myTest", {
  get() {
    return 2;
  },
});

console.log(test.myTest());

This substitution is not production material, but a simulation of mocking/stubbing on unit tests. Libraries like sinon do something like this to achieve the mocking. Notice that, in a real project, I’m using typescript register + mocha, not running directly the compiled code, but with this setup is easier to simulate the problem.

Expected behavior: I expect that the js code prints 2;

Actual behavior: In js alternative 1, It prints 1, in js alternative 2, it throws an error; The problem happens because of how the index.js is generated. Since 3.9.2, it is generated like this:

 
"use strict";
var __createBinding = (this && this.__createBinding) || (Object.create ? (function(o, m, k, k2) {
    if (k2 === undefined) k2 = k;
    Object.defineProperty(o, k2, { enumerable: true, get: function() { return m[k]; } });
}) : (function(o, m, k, k2) {
    if (k2 === undefined) k2 = k;
    o[k2] = m[k];
}));
var __exportStar = (this && this.__exportStar) || function(m, exports) {
    for (var p in m) if (p !== "default" && !exports.hasOwnProperty(p)) __createBinding(exports, m, p);
};
Object.defineProperty(exports, "__esModule", { value: true });
exports.myTest2 = void 0;
__exportStar(require("./my-test"), exports);
function myTest2() {
    return 3;
}
exports.myTest2 = myTest2;

Notice that, in the line 4 of this code, an accessor is defined. On version 3.8.3, it generates like this:

"use strict";
function __export(m) {
    for (var p in m) if (!exports.hasOwnProperty(p)) exports[p] = m[p];
}
Object.defineProperty(exports, "__esModule", { value: true });
__export(require("./my-test"));
function myTest2() {
    return 3;
}
exports.myTest2 = myTest2;

Notice that the function declared in the index.ts was exported correctly, following what is specified in the official documentation, but both generated codes actually are not in total compliance with it. The problem is that, with the latest version, it also changes the behavior, making read-only the noncompliant exporting, which can break code that relies on it being read-write, like mocking mechanism in unit tests, for example. The unit tests could also have a distinct tsconfig using modules = commonJs just to have such behavior allowing mocking, while the main tsconfig not.

Playground Link: I’m unable to create a playground example as this issue only happens using multiple files, but i created a example project: https://github.com/Farenheith/issue-readonly-exporting-example

Related Issues: this issue but it was marked as work as intended, probably because I didn’t notice that the transpiler was not following the compiler option module configuration the first time I opened it.

[Farenheith]

Same problem. I have to back to v3.8.3 to make sinon working, is there any progress in the follow-up?

@FeelyChau I found a way to make the tsc generated commonjs modules mockable again. It’s a workaround but works with version 4.1.3 of typescript! I created a package to make it easy to use: https://www.npmjs.com/package/mockable-ts-commonjs

[Farenheith]

@FeelyChau maybe using some rewire strategy https://stackoverflow.com/questions/38273619/using-sinonjs-stub-with-rewire is the best way to go. I don’t know if some mocking library have it already implemented, I think sinon does not