How Jest Reveals Test Smells

Unfortunately I discover very often when using jest that a lot of the things point me to smells in a test. Jest even encourages some of those instead of encouraging us to write better tests. I found some things over time, so I just collect them here.

Is there a blog post showing how all the features of jest can be prevented to be needed by better test writing practices?
Maybe I should write it.#javascript #testing

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) May 6, 2020

jest.mock

I just saw code that uses jest.mock and read https://t.co/JFpyTMojQn
I hear "let the tests drive you", I think "too much setup code".

I am trying to prevent to mock what I don't own.
I feel bad when I (rarely) test that the right parameter is passed to a stubbed function.

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) May 13, 2020

jest.fn

Check how many stubs/spies you can replace by noop-functions!

If you don't spy, don't use
- sinon.spy()
- jest.fn()
- ...

A noop-function does the job as well

- () => {}
or
- async () => {}

Don't make the #test more complicated than necessary!

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) June 24, 2020

jest.setSystemTime() or jest.getRealSystemTime()

You are just about to use `jest.setSystemTime()` or `jest.getRealSystemTime()`?
Why not inject the function that you are actually trying to fake a value for and inject this dependency?#dependencyInjection#explicitCode

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) December 7, 2020

How?

// dont use:
const fn= () => {
x = new Date().toISOString();
}

// use:
const fn = (nowFn) => {
x = nowFn();
}

and you can control what `nowFn` is, does and returns in your test, because you inject it.#tdd #testing #jest #mocha#inject #dependencies
2/

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) December 7, 2020

You can also inject a default dependency (for production), if you like:

const defaultNowFn =
() => new Date().toISOString();

const fn =
(nowFn = defaultNowFn) => {
...
}

3/#javascript #jest #tested #code #parameterized

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) December 7, 2020

Reintroduce this?

"You can also pass variables from this module to your test suites by assigning them to https://t.co/bpdtat1Io7 object – this will make them available in your test suites as global variables."https://t.co/xvNGQO0ywg

Wasn't the magic `this` one of the mistakes of mocha?#jest

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) May 19, 2020

Filter tests

This pattern of jest to let users filter the tests down easily and not running all tests feels like an anti-pattern to me.

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) November 4, 2020

Depdenency Injection

Another good piece of documentation which, if you properly listen to it whispering, tells you to inject dependencies that otherwise leak in.
Make them transparent, it hurts in the beginning, but pays off.https://t.co/CMGbRVChoW

see also https://t.co/2894cILEkk#jest

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) May 19, 2020

Jest Is Slow

Running the tests of one file with mocha takes 0.2s running the same tests with jest 1.5s ... why should I be waiting 1.3s more every time? What's the value added?

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) October 25, 2019

Various

Why do the tests fail when I have no test inside a test file?
A warning, ok, but it's not a test fail. I am an adult, please.

"Your test suite must contain at least one test."#jest

— Wolfram (JS-- HTML++ CSS++) Kriesing (@wolframkriesing) May 19, 2020