How to debug import of a javascript function in React?

Question

I have some functions in a javascript file (helpers.js) that are imported into a react file (caller.jsx).

helpers.js:

async function one() {
  // ...
}

function two() {
  // ...
}

module.exports = { one, two }

caller.jsx:

const { one, two } = require("../utils/helpers")

When I run my local React server (npm run start), everything works fine. I can use the app and it works.

When I run a production build (npm run build) and serve up the javascript from that build, it doesn't work, and there's an error message in the console:

main.d780c23e.chunk.js:1 Uncaught TypeError: Cannot assign to read only property 'exports' of object '#<Object>'
    at Module.<anonymous> (main.d780c23e.chunk.js:1)
    at Module.228 (main.d780c23e.chunk.js:1)
    at i ((index):3)
    at Module.375 (main.d780c23e.chunk.js:1)
    at i ((index):3)
    at t ((index):3)
    at Array.r [as push] ((index):3)
    at main.d780c23e.chunk.js:1

And the compiled code in the browser shows the problem at the exports of helpers.js, right at the equals sign (=):

            b.exports = {
                one: function() { ... },
                two: function() { ... }
                }
            }

I have also tried importing the symbols in caller.jsx:

import { one, two } from "../utils/helpers"

In that case, the production build complains that the symbol is not exported.

./src/hooks/caller.jsx
Attempted import error: 'two' is not exported from '../utils/helpers'.

And if I delete two, then it complains about one.

It seems the CommonJS style of code (with module.exports) isn't working in the ESModule style of code (with import).

However, I have some other files (call them caller2.jsx and helper2.js) where it is working for helper2.js to use module.export and caller2.jsx to use an import statement.

I am flummoxed about how to tell what's going in.

It IS exported, damnit! Why does only the production compiler say it's not?

Versions: node v16.10.0, npm 7.24.0, react 17.0.2, react-scripts 4.0.3.

EDIT: Some build info: we build using npm. Here are the scripts in package.json:

"scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build"
}

As an example, here is the full output of build:

$ npm run build

> myapp@0.1.0 build
> react-scripts build

Creating an optimized production build...
Compiled successfully.

File sizes after gzip:

  421.79 KB        build/static/js/2.9462f3d3.chunk.js
  23.57 KB         build/static/css/2.0a9ec390.chunk.css
  20.85 KB (+1 B)  build/static/js/main.2d21dfa5.chunk.js
  2.49 KB          build/static/css/main.8d569477.chunk.css
  1.73 KB          build/static/js/3.2618e91d.chunk.js
  1.17 KB          build/static/js/runtime-main.2616b467.js

The project was built assuming it is hosted at /.
You can control this with the homepage field in your package.json.

EDIT 2: We are using hardhat, so we cannot use ESModule exports. See for example this github issue, or this stackoverflow question.