startOf

Returns a new Date set to the start of the given unit.

• 'day' — 00:00:00.000
• 'month' — 1st of the month, 00:00:00.000
• 'year' — January 1st, 00:00:00.000

Returns null if the input is invalid.

Available since v2.0.0

Import

import { startOf } from '@helpers4/date';
// or, from the all-in-one package (same code, one install):
import { startOf } from 'helpers4/date';

Signature

startOf(date: DateLike, unit: DateTruncUnit): Date | null

Parameters

Parameter	Type	Description
date	DateLike	The base date
unit	DateTruncUnit	The unit to truncate to

Returns

Date | null — A new Date at the start of the unit, or null

Examples

startOf

startOf('2025-06-15T14:30:00Z', 'day')    // => 2025-06-15T00:00:00.000Z
startOf('2025-06-15T14:30:00Z', 'month')  // => 2025-06-01T00:00:00.000Z
startOf('2025-06-15T14:30:00Z', 'year')   // => 2025-01-01T00:00:00.000Z

Related Types

DateLike

A value that can be converted to a Date.

• Date — used as-is (validated for Invalid Date)
• number — treated as a timestamp (seconds or milliseconds, auto-detected); 0 is treated as invalid and produces null in ensureDate
• string — parsed via new Date(string)
• Temporal.Instant | Temporal.ZonedDateTime — read via their epochMilliseconds property

type DateLike = Date | number | string | Temporal.Instant | Temporal.ZonedDateTime

DateTruncUnit

Units supported by startOf and endOf.

type DateTruncUnit = 'day' | 'month' | 'year'

Source

View source on GitHub