endOfMonth

1/**
2 * Returns a new Date object set to the end of the month (last day, 23:59:59.999).
3 *
4 * @param date - The input date.
5 * @returns A new Date representing the end of the month.
6 */
7export function endOfMonth(date: Date): Date {
8  return new Date(date.getFullYear(), date.getMonth() + 1, 0, 23, 59, 59, 999);
9}

• Precise Month-End Boundary

  Sets the date to the last day of the month at 23:59:59.999, covering the full extent of the month without overshooting.

• Handles Leap Years & Month Length Variability

  Uses new Date(year, month + 1, 0) — a well-known pattern to get the last day of any month reliably (including February in leap years).

• Immutable Result

  Returns a new Date object, keeping the input untouched — safe for use in pure functions or pipelines.

• Local Time Awareness

  Aligns with user expectations in client-side applications, especially in filters and reports.

Tests | Examples

1test('endOfMonth - returns last day of month at 23:59:59.999', () => {
2  const input = new Date('2024-02-10T12:00:00.000Z'); // Leap year
3  const result = endOfMonth(input);
4
5  expect(result.getFullYear()).toBe(2024);
6  expect(result.getMonth()).toBe(1); // February
7  expect(result.getDate()).toBe(29); // Leap year
8  expect(result.getHours()).toBe(23);
9  expect(result.getMinutes()).toBe(59);
10  expect(result.getSeconds()).toBe(59);
11  expect(result.getMilliseconds()).toBe(999);
12});
13
14test('endOfMonth - does not mutate original date', () => {
15  const input = new Date('2023-04-20T08:00:00.000Z');
16  const copy = new Date(input.getTime());
17  endOfMonth(input);
18  expect(input).toEqual(copy);
19});

Common Use Cases

• Range Filters for Monthly Reports

  Define the inclusive upper boundary of a month when querying records or logs.

• Billing and Subscription Logic

  Set invoice or trial end dates to the precise end of a billing month.

• Calendar Navigation

  Highlight or scroll to the last day of a month in UI components.

• Data Cleanup or Archival Cutoffs

  Set precise boundaries for processing or migrating data based on month-end logic.

• Validation Logic

  Ensure no timestamp exceeds the end of the intended month.