startOfMonth

1/**
2 * Returns a new Date object set to the start of the month (1st, 00:00:00.000).
3 *
4 * @param date - The input date.
5 * @returns A new Date representing the start of the month.
6 */
7export function startOfMonth(date: Date): Date {
8  return new Date(date.getFullYear(), date.getMonth(), 1, 0, 0, 0, 0);
9}

1import { startOfMonth } from './startOfMonth';
2
3test('startOfMonth - returns first day of month at 00:00:00.000', () => {
4  const input = new Date('2023-08-25T15:30:00.000Z');
5  const result = startOfMonth(input);
6
7  expect(result.getFullYear()).toBe(2023);
8  expect(result.getMonth()).toBe(7); // August
9  expect(result.getDate()).toBe(1);
10  expect(result.getHours()).toBe(0);
11  expect(result.getMinutes()).toBe(0);
12  expect(result.getSeconds()).toBe(0);
13  expect(result.getMilliseconds()).toBe(0);
14});
15
16test('startOfMonth - does not mutate original date', () => {
17  const input = new Date('2023-08-25T10:00:00.000Z');
18  const copy = new Date(input.getTime());
19  startOfMonth(input);
20  expect(input).toEqual(copy);
21});