isLeapYear

1/**
2 * Determines whether the given year is a leap year.
3 *
4 * @param yearOrDate - The year as a number or a Date object.
5 * @returns True if it's a leap year, false otherwise.
6 *
7 * @example
8 * isLeapYear(2024) // true
9 * isLeapYear(new Date('2000-01-01')) // true
10 */
11export function isLeapYear(yearOrDate: number | Date): boolean {
12  const year = yearOrDate instanceof Date ? yearOrDate.getFullYear() : yearOrDate;
13
14  return (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0;
15}

1test('identifies leap year by number', () => {
2  expect(isLeapYear(2024)).toBe(true);
3});
4
5test('identifies non-leap year by number', () => {
6  expect(isLeapYear(2023)).toBe(false);
7});
8
9test('identifies leap year by Date object', () => {
10  expect(isLeapYear(new Date('2000-02-29'))).toBe(true);
11});
12
13test('identifies non-leap year by Date object', () => {
14  expect(isLeapYear(new Date('2100-01-01'))).toBe(false);
15});