Add support for spyOn() as a valid global

I am trying to use the spyOn() method in my Jest tests. Although this is a valid global Jest function, it is still causing the following ESLint error:

error  'spyOn' is not defined

I have followed the instructions to setup this plugin, including installing it as a devDependency, adding “jest” to my eslintrc plugins and adding “jest/globals” to the env object. ESLint still throws an error. I even tried extending “plugin:jest/recommended”, but that still throws the error.

What I find interesting is that there was a past commit (2984eb50c) that introduced support for spyOn as a valid global. However, there was soon after another commit (491271aaa) that reverted the previous one, so that support for spyOn was removed. I am curious to understand more about the intent of these commits. It seems like the former commit, which includes support for spyOn as a valid global, would fix my issue.

In the meanwhile, I am getting around this my adding a /* global spyOn */ comment to each file that uses spyOn(). Ideally, this project can make ESLint smart enough to know that spyOn is a valid global method.

Thank you.

[SimenB]

Using spyOn, pending, fail etc is not really supported.

That said, I’d be happy to have a rule saying “don’t use jasmine globals”, whilst adding this to our list of globals

[GeorgeTaveras1231]

I’ve also found that there are behavioral differences between spyOn and jest.spyOn.

In my test, I initially had:

spyOn(window.location, 'assign')

/* Omitted */

expect(window.location.assign).toHaveBeenCalledWith('/some-path')

Which worked fine.

After changing it to use the jest global, it stopped working

jest.spyOn(window.location, 'assign')

/* Omitted */

expect(window.location.assign).toHaveBeenCalledWith('/some-path')

This would yield the JSDOM error:

SyntaxError: Could not resolve the given string "/some-path" relative to the base URL "about:blank"

To fix this, I had to explicitly mock the implementation of the spy:

jest.spyOn(window.location, 'assign').mockImplementation(() => {}) // < Call .mockImplementation

/* Omitted */

expect(window.location.assign).toHaveBeenCalledWith('/some-path')

Conclusion

It seems like spyOn automatically mocks the implementation with a No-Op function, which allowed the original version of the test to work.

Hopefully with this I’ve demonstrated that there may be use cases for the global spyOn. However, it is also hard to tell whether this difference was actually intended by the Jest developers.