useInterval with watched returns

useInterval.test.tsx

import { renderHook } from "@testing-library/react";
import useInterval from "./useInterval";

describe("useInterval", () => {
  beforeEach(() => {
    jest.useFakeTimers();
  });

  afterEach(() => {
    jest.clearAllTimers();
    jest.useRealTimers();
  });

  it("should start the interval on mount when called by itself", () => {
    const callback = jest.fn();
    renderHook(() => useInterval(callback, 1000));
    jest.advanceTimersByTime(1000);
    expect(callback).toHaveBeenCalledTimes(1);
  });

  it("should start the interval on mount when return value is assigned", () => {
    const callback = jest.fn();
    renderHook(() => {
      const interval = useInterval(callback, 1000);
      return interval;
    });
    jest.advanceTimersByTime(1000);
    expect(callback).toHaveBeenCalledTimes(1);
  });

  it("should not start the interval on mount when return value at index 0 is extracted", () => {
    const callback = jest.fn();
    renderHook(() => {
      const [poll] = useInterval(callback, 1000);
      return poll;
    });
    jest.advanceTimersByTime(1000);
    expect(callback).not.toHaveBeenCalled();
  });

  it("should start the interval on mount when return value at index 1 is extracted", () => {
    const callback = jest.fn();
    renderHook(() => {
      const [, nullish] = useInterval(callback, 1000);
      return nullish;
    });
    jest.advanceTimersByTime(1000);
    expect(callback).toHaveBeenCalledTimes(1);
  });

  it("should not start the interval on mount, but on command when return value is extracted and called", () => {
    const callback = jest.fn();
    const { result } = renderHook(() => {
      const [poll] = useInterval(callback, 1000);
      return poll;
    });
    jest.advanceTimersByTime(1000);
    expect(callback).not.toHaveBeenCalled();

    result.current();
    jest.advanceTimersByTime(1000);
    expect(callback).toHaveBeenCalledTimes(1);
  });
});