How do I mock require('module/submodule')

[seekshiva]

It says in the doc that we can create mock for node modules by placing __mocks__ in the same directory as node_modules.

How do I mock require('module/submodule')?

I tried this:

/
  __mocks__/
    module/
      submodule.js

and this:

/
  __mocks__/
    module.js

where module.js would contain this:

module.exports = {
  submodule: () => {}
}

but neither of these seem to work.

[cpojer]

If your module is located at <dir>/module.js you should create a mock under <dir>/__mocks__/module.js.

[seekshiva]

@cpojer It is a npm module.

I do require("collections/sorted-map"). The docs say that when I need to mock the functionality of a npm module, the mocks need to be present in the same folder that contains node_modules.

I tried doing __mocks__/collections/sorted-map.js and __mocks__/collections.js. In the second case, collection.js would contain module.exports = {"sorted-map": () => {}}

But neither of those cases work.

[cpojer]

Are you using npm3?

[seekshiva]

Yes

[cpojer]

Unfortunately we don't have a good solution for this yet :( We aren't using npm3 at FB yet and I haven't been able to investigate this issue thoroughly yet. If you have an idea for a fix, I'd appreciate a pull request!

[seekshiva]

I'm trying to mock collections/sorted-map because of this issue: babel/babel-jest#41. I thought it would be an issue with babel-jest and not jest itself, so I filed an issue there.

[seekshiva]

Is there a development doc for Jest that I can take a look at?

[cpojer]

what kind of development doc are you looking for? I'm working on revamping the CONTRIBUTING guide. All you need to do is make a change and submit a PR on GitHub. Our bot will then ask you to sign the CLA.

You can run jest on itself by doing node bin/jest.js.

[Xesued]

For anyone else looking into this, you can now create a map with: moduleNameMapper.

Jest configuration:

{
  ...
  "moduleNameMapper": [
    "collections/sorted-map": "<rootDir>/__mocks__/collections/sorted-map.js"
    ...
    ]
}

Really, you can map it to any file.

[StevePotter]

@Xesued is correct but I was unable to use the same kind of naming. I was trying to mock a file called HeaderStyleInterpolator.s fromreact-navigation. This worked for me:

    "moduleNameMapper": {
      "./HeaderStyleInterpolator": "<rootDir>/js/testing/NavHeaderStyleInterpolatorMock.js"
    }

[jquense]

this should be re-opened. It's still a problem and mocking via moduleNameMapper is bad because it's impossible to require the real module from the mock file

[thernstig]

@cpojer @xeodou @jquense @StevePotter

I recently opened this issue: #11136

Reading this thread, it seems it is not possible to mock require('fs/promises') without using the Jest configuration moduleNameMapper? That is not a proper solution.

Since Node.js 15 and later actually has promises as the first go-to for the fs module, and they also recommend require('fs/promises') to be used, this feels more urgent to solve.

Should I re-phrase my issue at #11136 slightly, or keep it as is? Because it does not seem to be a doc problem only. It also is not possible to mock, right?

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