jest.fn's implementation argument is no longer optional

[fgrandel]

If you know how to fix the issue, make a pull request instead.

• I tried using the @types/jest package and had problems.
• I tried using the latest stable version of tsc. https://www.npmjs.com/package/typescript
• I have a question that is inappropriate for StackOverflow. (Please ask any appropriate questions there).
• Authors: @antoinebrault

This was introduced by PR #31094 / Commit 3b921ef

[antoinebrault]

What is the issue ? Do you have an example ?

current implementation is :

function fn(): Mock;
function fn<T, Y extends any[]>(implementation: (...args: Y) => T): Mock<T, Y>;

[fgrandel]

type PropOf = T[keyof T]
...
mockedMethods.forEach(mockedMethod => mocked[mockedMethod] = jest.fn<PropOf>() as MockFunction<T[keyof T]>)

(This already presumes the PR I just provided)

Before I either had the choice not being able to get proper typing at all or pass jest.fn(undefined as any) into jest.fn

[fgrandel]

Let me give you the full source code - otherwise it will be hard to understand what we're up to...

We have a small utility function that allows us to mock any typescript interface like this:

...
let mockMyInterface: Mocked<MyInterface>
mockMyInterface = mock<MyInterface>('someMethod', 'someOtherMethod')
...
mockMyInterface.someMethod.mockReturnValue('12345')
...

export type MockFunction<F extends GenericFunction> = F & jest.Mock<ReturnType<F>, ArgsType<F>>

export type Mockable<T> = {
  [K in keyof T]: GenericFunction
}

export type Mocked<T extends Mockable<T>> = {
  [K in keyof T]: MockFunction<T[K]>
}

type PropOf<T> = T[keyof T]

export function mock<T extends Mockable<T>>(...mockedMethods: (keyof T)[]): Mocked<T> {
  const mocked: Mocked<T> = {} as Mocked<T>
  mockedMethods.forEach(mockedMethod => mocked[mockedMethod] = jest.fn<PropOf<T>>() as MockFunction<T[keyof T]>)
  return mocked
}```

[antoinebrault]

Ok I understand. Thanks. I'll review the PR.