automocking ignores arrow-functions in ES6 classes

[vonovak]

This is something between a question and feature request:

I was checking out the docs for es6 class mocks. The example uses the following code:

// sound-player-consumer.js
import SoundPlayer from './sound-player';

export default class SoundPlayerConsumer {
  constructor() {
    this.soundPlayer = new SoundPlayer();
  }

  playSomethingCool() {
    const coolSoundFileName = 'song.mp3';
    this.soundPlayer.playSoundFile(coolSoundFileName);
  }
}

If I create an automock of such module, the playSomethingCool function will be mocked. However, if I change it to an arrow function (which I would wanna do because of the behavior of this), the mock won't contain the playSomethingCool function. I am aware of the fact that the arrow function is not present on the object's prototype which is probably why the behavior is different.

I would like to ask if this is the intended behavior and if it would make sense to create mocks also for the arrow functions. I feel like this is something that should be covered in the docs, which I wanted to do a PR for but I figured I would first ask here.

Thanks

running jest 22.4

[mjesun]

Yep, Jest can only mock the structure of objects that are present at require time. It does it by reflection (not by analysis), which means that properties that get added by the constructor cannot be mocked. It's important to understand though that a fat-arrow assignment in a class in JS is not a class method; it's a class property holding a reference to a function.

My suggestion to that would be that you change the fat arrow assignments to standard methods and bind them in the constructor. That way they will be Jest aware. Otherwise I'm afraid there's not much we can do here.

I'm happy to take a PR with some improvements to the documentation, though 🙂

[vonovak]

thanks @mjesun that's what I was thinking. I'll ship the PR soon.

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