src/classes/CalendarDay.test.ts

import { CalendarDay } from "./CalendarDay";

it("should set `displayMonth` to undefined when date and `displayMonth` are in the same month", () => {
  const date = new Date(2020, 0, 1);
  const displayMonth = new Date(2020, 0, 15);
  const day = new CalendarDay(date, displayMonth);
  expect(day.displayMonth).toEqual(displayMonth);
  expect(day.outside).toEqual(false);
});

it("should set `displayMonth` to the given `displayMonth` when date and `displayMonth` are not in the same month", () => {
  const date = new Date(2020, 0, 1);
  const displayMonth = new Date(2020, 1, 15);
  const day = new CalendarDay(date, displayMonth);
  expect(day.displayMonth).toEqual(displayMonth);
  expect(day.outside).toEqual(true);
});