things turning me on this week
☕enjoy with a hot cupa java/script
JavaScript has gotten even better and you can now run 100% ES Modules code “natively” through the entire node, JS, testing, and coverage (and more!) ecosystem. Picking the right tools that understand ES Modules is key - here we use mocha and c8 (also show how to use jasmine (what is under jest) and nyc/. istanbul if desired). Bonus points? sinon for mocking.

Sample code setup

demonstration minimal working git repository

Uses simply: mocha and c8

The goal is to avoid babel, transpiling, and any dynamic transform of the JS source/test code as possible.

• mocha since it can understand ES Modules (and make import/export work nicely).
• c8 for code coverage (since it understands ES Modules)
• expectations for jest/jasmine like testing, eg: expect('hai'.length).toBe(3)
• sinon for mocking, stubs, and spies (more below on that)

There’s a few subtle things to get setup and “just right” to make it all work seamlessly. But once you know them, you’re good to go.

If you prefer jasmine

• see this prior setup

Thus

you can now write all your source and test code in the modern/future default 100% ES Modules (AKA import / export), eg:

import { foo, foobar } from '../src/foo.js'

describe('foo()', function () {
  it('should return number plus one', function () {
    expect(foo(1)).toEqual(2)
  })
})

run

git clone https://github.com/traceypooh/js-es-modules-coverage-testing
cd js-es-modules-coverage-testing
npm install
npm test

(or alternatively: npx c8 mocha)

yields

> @ test /Users/tracey/dev/js-es-modules-coverage-testing
> c8 mocha

  bar()
    ✓ should return number minus one

  foo()
    ✓ should return number plus one

  foobar()
    ✓ should return same number

  goo()
    ✓ should switch from 2 to 3 via a swap out
    ✓ should switch from 2 to 3 via a mock

  5 passing (11ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |
 bar.js   |     100 |      100 |     100 |     100 |
 foo.js   |     100 |      100 |     100 |     100 |
 goo.js   |     100 |      100 |     100 |     100 |
----------|---------|----------|---------|---------|-------------------

the magic

• in package.json, this tells node and scripts all .js files are ES Modules

{
  "type": "module",
}

• in package.json, this tells mocha to preload expectations (for jest-like testing)

{
  "mocha": {
    "require": "expectations"
  }
}

mocking, stubs, and spies

By Far, this is the most complicated thing to get right. I spent hours and hours over a few days on/off trying to get mocking to work (first with jasmine, then sinon).

Fortunately, I found this lil’ gem:

• https://stackoverflow.com/a/46739544

Normally, ES Modules are very locked down and hard to mock. By grouping all the export-ed methods via export default Goo, and importing via import Goo from '../src/goo.js', we can replace methods if needed for testing. (Imagine needing to test something that live-fetches resources, etc.)

• see src/goo.js
• see test/goo.test.js

2020 as ever, is a super-exciting time 🥳 for javascript!