Proposal for jest.mock to assist in typescript environments

[Paratron]

🚀 Feature Proposal

It would be super helpful if jest.mock could return the module (maybe with updated typings).

Motivation

Right now, when writing unit tests with jest and typescript, mocking modules becomes boilerplatey. This is because the IDE does not understand that after calling jest.mock("myModule") all exported functions of that module are of type jest.Mock. Therefore, when I try to call methods like myModule.mockReturnValue() or the likes, the IDE / typescript complains that this is nonsense - those functions do not exist on the module/function.

In order to get correct typings, one has to import a mocked module like this:

jest.mock("node-fetch");
import {default as fetchOriginal} from "node-fetch";

const fetch = fetchOriginal as unknown as jest.Mock;

Example

My idea is to let jest.mock return an instance of the mocked module - in the best case with correct typings. So the whole boilerplate could be reduced to:

const fetch = jest.mock("node-fetch");

Even if its not possible to add the correct typings directly in jest, it would make re-assigning mock types much easier:

const fetch = jest.mock("node-fetch") as unknown as jest.Mock;

Or when importing just some features of a module:

const {a, b} = jest.mock("myModule") as unknown as {a: jest.Mock, b: jest.Mock};

Thats possibly a problem since I saw that jest.mock currently returns an instance of jest for method chaining. So changing the behaviour would possibly break some peoples unit tests. Adding a property to the config argument could change that or maybe introducing a completely new method that combines mock + require.

[ahnpnl]

https://kulshekhar.github.io/ts-jest/docs/test-helpers#mocked

[SimenB]

We cannot change jest.mock to have any return-value but the current jest (for chaining) as it's hoisted by the babel plugin.

Also, jest.mock today doesn't actually mock the module right away, it just registers the fact that this module will be mocked whenever it is required.

Thirdly, there is no way in TypeScript to infer shape of an import except via typeof import('blahblah') (with a string literal) - so we'd either need to accept a generic or you will need to keep casting.

All that is to say there'd still be some boilerplate and we'd need a new API in order to implement this. Not blockers of course, but a significant effort.

---

The mocked helper @ahnpnl links solves half your issue I think (getting correct types), but still quite boilerplatey. I'm not sure if it's worth introducing an entire new API in order to reduce 3 lines into 1, tho... You can do something like

const fetch = mockAndRequire<typeof import('node-fetch')>('node-fetch');

function mockAndRequire<T = unknown>(moduleToImport: string): T {
  jest.mock(moduleToImport)

  const importedModule = require(moduleToImport);

  return mocked(importedModule, true)
}

or for ESM (though blocked on #10025 for now)

const fetch = await mockAndImport<typeof import('node-fetch')>('node-fetch');

async function mockAndImport<T = unknown>(moduleToImport: string): Promise<T> {
  jest.mockModule(moduleToImport)

  const importedModule = await import(moduleToImport);

  return mocked(importedModule, true)
}

Still some boilerplate (you have to say 'node-fetch' twice), but I don't think that's avoidable.

[wh1t3cAt1k]

I am successfully using https://github.com/userlike/joke for "mock and import" behaviour. In a recent version, they actually use the "mocked" helper mentioned above for typings, so far, I have not encountered any problems with the types whatsoever.

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