Partial types for mocked return values

[jfrs]

Clear and concise description of the problem

As a developer using Vitest I want to be able to mock only certain properties or functions of a module so that I can keep the type checking from TypeScript but only mock what I need for the test.

Currently when using vi.mocked and mockReturnValue with TypeScript it will expect a fully compatible type, but most of the time since you're mocking you only need some of the properties to be set.

This is an issue I face a lot when mocking but I haven't found a proper solution, the most common solution to this seems to be casting to unknown and but then you lose the type checking.

Suggested solution

There's a solution suggested here but I think just changing the return type to be a Partial<T> would work?

(Copied suggestion just in case:)

type DeepPartial<T> = {
    [P in keyof T]?: T[P] extends Array<infer U>
        ? Array<DeepPartial<U>>
        : T[P] extends ReadonlyArray<infer U>
            ? ReadonlyArray<DeepPartial<U>>
            : DeepPartial<T[P]>
};

type PartialReturnType<T extends MockableFunction> = 
    ReturnType<T> extends Promise<infer U>
        ? Promise<DeepPartial<U>>
        : ReturnType<T> extends Array<infer P>
            ? Array<DeepPartial<P>>
            : DeepPartial<ReturnType<T>>;

interface MockWithArgs<T extends MockableFunction> extends jest.MockInstance<PartialReturnType<T>, ArgumentsOf<T>> {
    new (...args: ConstructorArgumentsOf<T>): T;
    (...args: ArgumentsOf<T>): PartialReturnType<T>;
}

Alternative

I use @ts-ignore so I can keep the types but disable the error.

There's also this related issue but it solves a different problem I think.

Additional context

My current use case for this is mocking Pinia stores:

vi.mocked(useAuthStore).mockReturnValue({
  loggedIn: true
});

This will produce an error because the return value is missing a lot of Store properties and functions.

Validations

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• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[sheremet-va]

I don't see us "just" changing type to partial, since someone might rely on it.

I am not against partiallyMocked utility function thought.