startOfDay

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

1test('startOfDay - resets time components', () => {
2  const input = new Date('2023-08-25T14:45:30.500Z');
3  const result = startOfDay(input);
4
5  expect(result.getFullYear()).toBe(2023);
6  expect(result.getMonth()).toBe(7); // August
7  expect(result.getDate()).toBe(25);
8  expect(result.getHours()).toBe(0);
9  expect(result.getMinutes()).toBe(0);
10  expect(result.getSeconds()).toBe(0);
11  expect(result.getMilliseconds()).toBe(0);
12});
13
14test('startOfDay - does not mutate original date', () => {
15  const input = new Date('2023-08-25T14:00:00.000Z');
16  const copy = new Date(input.getTime());
17  startOfDay(input);
18  expect(input).toEqual(copy); // original date remains unchanged
19});