Jesm

Native ESM testing and mocking, jest-free:

• to mock ESM modules we are using quibble
• to use additional features like spies and stubs you can use sinon
• to use assertions you can use expect

Assumptions and limitations:

• nodejs >= 16 is required
• only test, it, describe, beforeAll, beforeEach, afterEach, afterAll, mockModule are supported (exported)
• only testNamePattern, runTestsByPath flags are supported
• only sequential running of test is supported (implicit --runInBand)
• no describe nested in describe support
• no coverage support
• jesm command must be run in the root of the package

Usage

Run spec files:

yarn add --dev jesm
NODE_OPTIONS="--loader=quibble" yarn jesm *.spec.js

See math.spec.js example on how to use mockModule.

mockModule

mockModule function can mock a module in 3 ways:

• inline providing of default export and named exports
• lookup of .mock.js file
• lookup of __mocks__ folder

Examples:

test('inline mock', async () => {
  // mock import
  await mockModule('./subtract.js', (a, b) => a * b, { MAGIC_NUMBER: 1 });

  // import mock, either a module importing the one mocked or importing directly the mocked one 
  const { subtract } = await import('./math.js');
  const { MAGIC_NUMBER } = await import('./subtract.js');

  const actual = subtract(2, 2);
  const expected = 4;
  expect(actual).toEqual(expected);
  expect(MAGIC_NUMBER).toEqual(666);
});

test('.mock.js file mock', async () => {
  // mock import
  await mockModule('./multiply.js');

  // import mock
  const { multiply } = await import('./math.js');
  const { MAGIC_NUMBER } = await import('./multiply.js');

  const actual = multiply(3, 3);
  const expected = 27;
  expect(actual).toEqual(expected);
  expect(MAGIC_NUMBER).toEqual(2);

  // reset mocks
  mockModule.reset();
});

test('__mocks__ folder mock', async () => {
  // mock import
  await mockModule('./divide.js');

  // import mock
  const { divide } = await import('./math.js');
  const { MAGIC_NUMBER } = await import('./divide.js');

  const actual = divide(3, 3);
  const expected = 27;
  expect(actual).toEqual(expected);
  expect(MAGIC_NUMBER).toEqual(2);

  // reset mocks
  mockModule.reset();
});

Webstorm support

Copy the .jest-run folder inside your root before running any spec with the run/debug button. Remember to properly set your NODE_ENV=xxx variable.

Notes

Thanks to Gil Tayar and its blog post here.