ESM internal module mocks

index.js

import { __testVisible as self } from './index.mjs';

function a() {
  return 1;
}

export function b() {
  return self.a();
}

export const __testVisible = {
  a,
};

index.test.js

import chai from 'chai';
import sinon from 'sinon';
import { b, __testVisible } from './index.mjs';

describe('mocks', function () {
  before(function () {
    sinon.stub(__testVisible, 'a');
  });

  afterEach(function () {
    sinon.reset();
  });

  it('calls through', function () {
    __testVisible.a.callThrough();
    chai.expect(b()).eql(1);
  });

  it('mocks', async function () {
    __testVisible.a.returns(2);
    chai.expect(b()).eql(2);
  });
});

output

❯ npx mocha index.test.mjs


  mocks
    ✔ calls through
    ✔ mocks


  2 passing (4ms)