导入类：

import { DateService } from "./DateService";

设置描述块并示例化DateService

describe("DateService", () => {
  const dateService = new DateService();

然后测试结果：

it("should return the number of days", () => {
  const month = new Date(2022, 4); // May 2022
  const daysInMonth = dateService.getDaysInMonth(month);
  expect(daysInMonth).toBe(31);
});

jest
  .useFakeTimers()
  .setSystemTime(new Date('2020-01-01'));

describe("DateService", () => {
  test("getDaysInmonth", () => {
    // arrange
    const dateService = new DateService();
    const expected = new Date('2020-01-01');

    // act
    const actual = dateService.getDaysInMonth();
    
    // assert
    expect(actual).toEqual(expected);
  }
})