Is your feature request related to a problem? Please describe.
Hard to unit test in isolation

Describe the solution you'd like
Ability to mock functions and get proper type for example

const double: (n: number): number => n * 2;
const doubleMock: ((n: number) => number) & jest.Mock  = mockIt(double);

// such that doubleMock extends the jest.mock type so this 👇 works
doubleMock.mockReturnValue(12);

Note: Should work for mocking modules, classes and individual functions.

Describe alternatives you've considered
Not having to do (scale.scale as jest.Mock) to mock the functions

describe('getPosition', () => {
  // @ts-ignore
  const scale: Scale = {
    scale: jest.fn(),
  };

  beforeEach(() => {
    (scale.scale as jest.Mock).mockClear();
  });

  it('should return value from scale', () => {
    (scale.scale as jest.Mock).mockReturnValue(20);
    const result = getPosition(10, scale);
    expect(result).toBe(20);
  });

  it('should call scale with correct args', () => {
    getPosition(10, scale);
    expect(scale.scale).toBeCalledWith(10);
  });
});

Unit test coverage is very good but I feel like the current tests don’t fully cover each scenario and enforce bug fixes. Every bug that is fixed, in my mind, should have a unit test that enforces that use case.

There is also a lack of component test coverage.

There is a lot of boilerplate code that is used to make assertions for just one use case then repeated for another. I think this could be DRY’d up but creating helper functions as well as default configurations constants that are then changed to make a given assertion. This can then be enhanced using property based testing via jsverify or others.

In my last job, I had a difficult time testing code in typescript as the assertions require each type to be fully built out or it will complain. Using ts-ignore is a useful workaround but employing helper functions to make partial assertions would be very beneficial. This is most common with mocked functions. I don’t care about the type I just want to assert that the function is called with the given arguments. Mocks are a MUST in jest testing and they are seldom used requiring a full build out of required parameters to make a given assertion.

Mocking canvas

Mocking the canvas element in unit tests is a good way to make assertions on the api level but it’s hard to capture all that a visual testing system would.