How can I use modules with mocha ? My mjs files get an error

Question

With commonjs and using require() for .js files and a command of

mocha `**/*.spec.js` 

Files:

capitalize.js
test/capitalize.js

my tests run

Output:

mocha
Capitalize sentences
✓ calls the function
✓ calls the function
✓ calls the function
✓ Works for a second sentence
✓ Handles 3 dots
✓ Handles existing capitalization dots
✓ Preserves existing last dot
✓ Preserves carriage returns ("\n")

I want to use ES6 modules. So I:

• changed both file extensions to `.js from .mjs

• changed file I an running (the test) from

  export function capitalize(paragraph) {
  

  to

  exports.capitalize = function(paragraph) {
  

• changed file I include from

  export function capitalize(paragraph) { 
  

  to

  exports.capitalize = function(paragraph) { 
  

However, mocha isn't recognizing my tests and I get

Error: No test files found: "**/*.spec.js"

// Not too surprising given I changed the file extensions

If I now changed package.json from

"test": "mocha **/*.spec.js"

to

"test": "mocha **/*.spec.mjs"

I get

$ mocha Error: No test files found

from mocha directly, or with npm test I get

Error [ERR_REQUIRE_ESM]: Must use import to load ES Module:

How to fix?