mobx: Mobx 6.1 breaks spying on actions in unit test

Intended outcome:

In our unit tests we are using the jest.spyOn() function to ensure that certain actions are called under certain conditions (with expected values) in our components.

Actual outcome:

With the 6.1 release changes all of our spys are failing in the same manner as below

setUsernameSpy = jest.spyOn(authStore, 'setUsername');

results in:

TypeError: Cannot assign to read only property 'setUsername' of object '[object Object]'

How to reproduce the issue: I’m pretty sure it’s obvious how to reproduce the issue. My question is HOW can I verify that an action on an observable is called with the proper arguments now that 6.1 is available? Is there an option I can set to turn off making actions read-only for tests?

Versions 6.0.4 - working 6.1 - not working

About this issue

Original URL
State: closed
Created 3 years ago
Reactions: 6
Comments: 16 (3 by maintainers)

Most upvoted comments

Depends if it’s plain object or not. All annotated fields of non-plain objects (classes) are non-configurable as well. We will probably end up with configure({ safeDescriptors: false }) or something like that. Actually already working on it, but still considering alternatives… EDIT: Note that in case of plain objects, changing property configuration via defineProperty is also potentially dangerous, but not forbidden because we have to support delete key/remove(key) and also it’s not something that can happen just by mistake (as opposed to classes).

urugator on Jan 28, 2021

@urugator I think being able to mock/spy actions is good enough. I cannot think about a use case where mocking an observable or computed is necessary (or even good practice?).

I’ve being writing MobX specs in large code bases for over a year and never had the need to mock an observable or computed.

Unfortunately version 6.1 broke many specs so upgrading is not an option for us till this is fixed. Thanks for your work!

We would like to know if it’s only about writability of actions or whether we should also expect demand for configurability of actions or other fields (computed/observable).

josuemontano on Jan 29, 2021

@ljani #2766 should fix this

urugator on Jan 29, 2021

@urugator

We would like to know if it’s only about writability of actions or whether we should also expect demand for configurability of actions or other fields (computed/observable).

It might be helpful:

Jest should be able to mock non-writable but configurable properties. Currently jest will throw an error when trying to replace non-writable (but configurable) properties.

https://github.com/facebook/jest/issues/8656

vkrol on Jan 28, 2021