Improve typing of isMockFunction

[tkalliom]

🚀 Feature Proposal

Constrain the signature of isMockFunction<T> to accept only functions returning T, removing the need to explicitly specify the type.

Motivation

The ts-jest package offers a mocked helper to type safely set mock implementations. However, that relies on the @types/jest package, which offers types defined but not exposed by Jest. (For example, at the time of writing @types/jest is still for Jest 25, so migration to Jest 26 is not possible for any TypeScript tests which mock modules)

At the moment, code like JestMock.isMockFunction<number>(sum) && sum.mockReturnValueOnce(5) is possible. However, that does not ensure that the type of the function matches the explictly specified one – so isMockFunction<string>(sum) && sum.mockReturnValueOnce("k") also transpiles. If the signature of isMockFunction was tightened to isMockFunction<T>(fn: (args: any) => T): fn is Mock<T>;, the combination of the signature and the type predicate would provide nice safety.

Example

With the aforementioned signature:

import { sum } from "arithmetic-utils";
import * as JestMock from "jest-mock";

jest.mock("arithmetic-utils");

// this is OK
JestMock.isMockFunction(sum) && sum.mockReturnValueOnce(5);
// this would be caught by transpiler
JestMock.isMockFunction(sum) && sum.mockReturnValueOnce("k");

Pitch

The MockInstance and Mock types are not exposed by jest-mock, apart from being referenced in function signatures. Therefore the same type safety can not be achieved by third-party libraries unless they duplicate those types, which is obviously fragile. Alternatives to the proposed solution would be to export MockInstance from jest-mock (making it easier to maintain compatibility of ts-jest with future Jest versions), or to provide an equivalent of mocked in Jest core (which would make things easy for app developers, but perhaps it would be considered out-of-scope for core).

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