diffBy

1/**
2 * Returns the difference between two dates by a specified unit.
3 *
4 * @param date1 - The first date.
5 * @param date2 - The second date.
6 * @param unit - The unit to compare by ('year', 'month', 'week', etc).
7 * @returns The signed difference in the specified unit.
8 */
9export function diffBy(
10  date1: Date,
11  date2: Date,
12  unit: 'year' | 'month' | 'week' | 'day' | 'hour' | 'minute' | 'second' | 'millisecond'
13): number {
14  const ms1 = date1.getTime();
15  const ms2 = date2.getTime();
16  const diffMs = ms2 - ms1;
17
18  switch (unit) {
19    case 'millisecond':
20      return diffMs;
21    case 'second':
22      return diffMs / 1000;
23    case 'minute':
24      return diffMs / (1000 * 60);
25    case 'hour':
26      return diffMs / (1000 * 60 * 60);
27    case 'day':
28      return diffMs / (1000 * 60 * 60 * 24);
29    case 'week':
30      return diffMs / (1000 * 60 * 60 * 24 * 7);
31    case 'month':
32      return (
33        (date2.getFullYear() - date1.getFullYear()) * 12 +
34        (date2.getMonth() - date1.getMonth())
35      );
36    case 'year':
37      return date2.getFullYear() - date1.getFullYear();
38    default:
39      throw new Error(`Unsupported unit: ${unit}`);
40  }
41}

1test('diffBy - year difference', () => {
2  expect(diffBy(new Date('2020-01-01'), new Date('2023-01-01'), 'year')).toBe(3);
3  expect(diffBy(new Date('2023-01-01'), new Date('2020-01-01'), 'year')).toBe(-3);
4});
5
6test('diffBy - month difference', () => {
7  expect(diffBy(new Date('2022-01-01'), new Date('2022-04-01'), 'month')).toBe(3);
8  expect(diffBy(new Date('2022-04-01'), new Date('2022-01-01'), 'month')).toBe(-3);
9});
10
11test('diffBy - week difference', () => {
12  expect(diffBy(new Date('2023-06-01'), new Date('2023-06-15'), 'week'))
13    .toBeCloseTo(2);
14  expect(diffBy(new Date('2023-06-01'), new Date('2023-06-18'), 'week'))
15    .toBeCloseTo(2.4285, 4);
16  expect(diffBy(new Date('2023-06-18'), new Date('2023-06-01'), 'week'))
17    .toBeCloseTo(-2.4285, 4);
18});
19
20test('diffBy - day difference', () => {
21  expect(diffBy(new Date('2023-06-01'), new Date('2023-06-05'), 'day')).toBe(4);
22});
23
24test('diffBy - hour difference', () => {
25  expect(diffBy(
26    new Date('2023-06-01T00:00:00'),
27    new Date('2023-06-01T03:00:00'),
28    'hour'
29  )).toBe(3);
30});
31
32test('diffBy - minute difference', () => {
33  expect(diffBy(
34    new Date('2023-06-01T00:00:00'),
35    new Date('2023-06-01T00:45:00'),
36    'minute'
37  )).toBe(45);
38});
39
40test('diffBy - second difference', () => {
41  expect(diffBy(
42    new Date('2023-06-01T00:00:00'),
43    new Date('2023-06-01T00:00:05'),
44    'second'
45  )).toBe(5);
46});
47
48test('diffBy - millisecond difference', () => {
49  expect(diffBy(
50    new Date('2023-06-01T00:00:00.000'),
51    new Date('2023-06-01T00:00:00.250'),
52    'millisecond'
53  )).toBe(250);
54});