How to mock and test submodules with Jest

[lundn]

Hi there!

I'm trying to write a test for a utility function in our project, which uses the formatDuration module from date-fns/formatDuration. Can anyone help me creating a mock for this module with Jest and be able to test if formatDuration was called with expected arguments?

This is my current setup:

// ../utilites/date.ts
import formatDuration from "date-fns/formatDuration";
import subSeconds from "date-fns/subSeconds";
import intervalToDuration from "date-fns/intervalToDuration";

export const createDurationFromSeconds = (seconds: number) => {
  const end = new Date();

  const start = subSeconds(end, seconds);
  return intervalToDuration({ start, end });
};

type FunctionArgs = Parameters<typeof formatDuration>;

export const formatDurationHelper = (
  duration: FunctionArgs[0],
  options?: FunctionArgs[1]
) => {
  //...do something with options.format

  return formatDuration(duration, options);
};

// ../utilites/date.test.ts
import formatDuration from "date-fns/formatDuration";
import { formatDurationHelper, createDurationFromSeconds } from "./date";

jest.mock("date-fns/formatDuration", () => {
  return {
    __esModule: true,
    default: jest.fn()
  };
});

describe("formatDurationWithLocale", () => {
  it("created mocked formatDuration", () => {
    expect(jest.isMockFunction(formatDuration)).toBe(true);  // <- this test is successful
  });

  it("does something with options.format", () => {
    const duration = createDurationFromSeconds(1);
    formatDurationHelper(duration, {
      format: ["seconds"]
    });

    expect(formatDuration).toHaveBeenCalled();  // <- this test is unsuccessful
    expect(formatDuration).toHaveBeenCalledWith(duration, {   // <- this test is unsuccessful
      format: ["seconds"]
    });
  });
});

expect(jest.isMockFunction(formatDuration)).toBe(true) is successful, but the others fail...

Thanks in advance!