spyOn getter requires explicit type on getter after Jest 24

[pburkindine]

🐛 Bug Report

I am trying to upgrade our repos to jest 24 and encountered a compilation issue with spying on a mocked getter which worked in jest 23. The invocation is:

jest.spyOn(someMock, 'someProp', 'get').mockReturnValue(someStub);

... and the error is:

Argument of type '"someProp"' is not assignable to parameter of type 'never'.

I can rectify the situation by explicitly setting the return type of the mock function, but it doesn't seem like that should be necessary.

To Reproduce

Steps to reproduce the behavior:

Does not work:

class TestA {
  get foo() {
    return jest.fn();
  }
}

const mock = new TestA();
jest.spyOn(mock, 'foo', 'get').mockReturnValue('baz);

Does not work:

class TestA {
  get foo(): jest.Mock {
    return jest.fn();
  }
}

const mock = new TestA();
jest.spyOn(mock, 'foo', 'get').mockReturnValue('baz);

Works:

class TestA {
  get foo(): {} {
    return jest.fn();
  }
}

const mock = new TestA();
jest.spyOn(mock, 'foo', 'get').mockReturnValue('baz);

Works:

class TestA {
  get foo(): any {
    return jest.fn();
  }
}

const mock = new TestA();
jest.spyOn(mock, 'foo', 'get').mockReturnValue('baz);

Expected behavior

A jest.fn() being returned does not appear to satisfy the condition [K in keyof T]: T[K] extends (...args: any[]) => any in NonFunctionPropertyNames

If it is necessary to explicitly cast jest.fn() return from mocks, this should be reflected in the documentation.

Run npx envinfo --preset jest

Paste the results here:

  System:
    OS: macOS High Sierra 10.13.6
    CPU: (12) x64 Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
  Binaries:
    Node: 10.15.0 - ~/.nvm/versions/node/v10.15.0/bin/node
    Yarn: 1.13.0 - /usr/local/bin/yarn
    npm: 6.8.0 - ~/.nvm/versions/node/v10.15.0/bin/npm
  npmPackages:
    jest: ^24.5.0 => 24.5.0 

[SimenB]

This isn't an error with jest, this is a type error from typescript. See @types/node where your types come from

[pburkindine]

@SimenB It seems like this issue was introduced by this commit on Feb 10, which introduced the NonFunctionPropertyNames type to spyOn in jest-mock:

00e52eb

This was working fine in Jest 23 and only occurs when returning a jest.fn(); I don't understand why that would not satisfy the new condition ((...args: any[]) => any) unless there is some foo surrounding the jest.fn() object

If you can please help me understand a bit better why this condition might be failing, I can file an issue with TS.

[SimenB]

What you're getting is a type error. While Jest has indeed migrated to TS internally, we still don't distribute our types (it's probably confusing since we copied a lot of the types in jets-mock from @types/jest, so the code is very similar). Maybe it's DefinitelyTyped/DefinitelyTyped@d4fe875#diff-4a4390f0b4b1f7ac085a3d127ce47adb? You can look through https://github.com/DefinitelyTyped/DefinitelyTyped/commits/master/types/jest/index.d.ts and find it

However, if it's undesirable (I don't know) we should definitely change the types in Jest itself as well. I can reopen to track this, but there shouldn't really be a code change here until it's at least been discussed in the context of @types/jest

[Godrules500]

I too am running into this error. Rather I have a type on the method or not I get the same errors. I'm on Jest and "@types/jest": "^24.0.18", and "jest": "^24.9.0" in an Angular environment.

Argument of type '"config"' is not assignable to parameter of type '"prototype"'
Argument of type '"appConfig"' is not assignable to parameter of type 'never'.

[hantsy]

After upgrading my nestjs project, I got similar error like this in jest testing codes. Jest version is 26.6.3.

 jest.spyOn(model, 'create').mockResolvedValue(toReturned);
                                                          ~~~~~~~~~~
    src/post/post.service.spec.ts:281:58 - error TS2345: Argument of type 'any' is not assignable to parameter of type 'never'.

    281     jest.spyOn(commentModel, 'create').mockResolvedValue({
                                                                 ~
    282       ...comment,
        ~~~~~~~~~~~~~~~~~
    283       post: { _id: 'test' },
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    284     } as any);

The create method is from Mongoose 5.12.0.

[hantsy]

I use mockImplementation(()=>Promise.resovle()) to resolve the problem temporarily. I think it is a problem of the incompatibility between @types/jest and Mongoose 5.12 to make it resolve the wrong type.

PS: Mongoose 5.11 introduce its own types instead of the @types/mongoose

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