Calculate age in years

get-age.ts

/**
 *
 */
export function getAge(now: Date, birthday: Date): number {
  // trick to ignore year
  const backToFuture = new Date(now.getTime());
  backToFuture.setFullYear(birthday.getFullYear());
  
  // 'now' and 'dateNoYear' has the same year, so it wouldn't affect the result
  // check month to avoid leap year issue
  const hasBirthdayThisYear = now.getMonth() >= birthday.getMonth() && backToFuture.getTime() >= birthday.getTime();
  return (now.getFullYear() - birthday.getFullYear()) + (hasBirthdayThisYear ? 0 : -1);
}



  /**
   *
   */
  describe('getAge', () => {

    it('already has a birthday this year', () => {
      // already has a birthday this year
      expect(getAge(new Date('2020-05-08T12:00'), new Date('2010-01-01T12:00'))).toBe(10);
      expect(getAge(new Date('2020-05-08T12:00'), new Date('2010-05-01T12:00'))).toBe(10);
    });
    it('not has a birthday this year', () => {
      // not has a birthday this year
      expect(getAge(new Date('2020-05-08T12:00'), new Date('2010-06-01T12:00'))).toBe(9);
      expect(getAge(new Date('2020-05-08T12:00'), new Date('2010-12-01T12:00'))).toBe(9);
    });
    it('zero age', () => {
      expect(getAge(new Date('2020-05-08T12:00'), new Date('2020-04-01T12:00'))).toBe(0);
    });

    it('leap year #1', () => {
      expect(getAge(new Date('2021-02-28T12:00'), new Date('2020-02-29T12:00'))).toBe(0);
    });

    it('leap year #2', () => {
      expect(getAge(new Date('2020-02-29T13:00'), new Date('2019-03-01T11:00'))).toBe(0);
    });

    it('leap year #3', () => {
      expect(getAge(new Date('2020-02-29T13:00'), new Date('2016-03-01T11:00'))).toBe(3);
      expect(getAge(new Date('2020-02-29T13:00'), new Date('2016-02-29T11:00'))).toBe(4);
    });
  });