How to resolve Flow type error from Jest mocking

Question

I am using Jest to mock certain functions from a module and to test in the following manner:

jest.mock("module", () => ({
  funcOne: jest.fn(),
  funcTwo: jest.fn(),
  ...
}));

import {funcOne, funcTwo, ...} from "module";

test("something when funcOne returns 'foo'", () => {
  funcOne.mockImplementation(() => 'foo');  // <- Flow error
  expect(...)
});

test("that same thing when funcOne returns 'bar'", () => {
  funcOne.mockImplementation(() => 'bar');  // <- Flow error
  expect(...)
});

How can I stop Flow from reporting a property 'mockImplementation' not found in statics of function error without error suppression (e.g. $FlowFixMe)?

I understand that the issue comes from the fact that the functions defined in the module are not Jest-mocked functions and, as far as Flow is concerned, do not contain methods like mockImplementation, mockReset, etc.

Answer 1

Thanks, Andrew Haines, the comments on the related issue you posted provides a solution. I am satisfied with the following:

const mock = (mockFn: any) => mockFn;

test("something when funcOne returns 'foo'", () => {
    mock(funcOne).mockImplementation(() => 'foo');  // mo more flow errors!
    ...
});

Answer 2

You can loosen the type constraint inline as well:

test("something when funcOne returns 'foo'", () => {
    (funcOne: any).mockImplementation(() => 'foo');  // mo more flow errors!
    ...
});