getISOWeek()

Returns the ISO 8601 week number (1–53) for a given date. ISO weeks start on Monday, and week 1 is the week containing the first Thursday of the year.

Syntax

getISOWeek(date)
getISOWeek(year, month, day)

Parameters

Parameter	Type	Required	Description
date	Date	Yes (overload 1)	The date object to examine
year	number	Yes (overload 2)	The Gregorian year (1–9999)
month	number	Yes (overload 2)	The month (1–12)
day	number	Yes (overload 2)	The day (1–31)

Returns

number - The ISO week number (1–53) for the given date

Basic Examples

Using a Date Object

import { getISOWeek } from 'date-and-time';

getISOWeek(new Date(2025, 0, 1));  // => 1  (Jan 1, 2025 is in week 1)
getISOWeek(new Date(2025, 5, 15)); // => 24 (Jun 15, 2025 is in week 24)

Using Year, Month, and Day

import { getISOWeek } from 'date-and-time';

getISOWeek(2025, 1, 1);  // => 1
getISOWeek(2025, 6, 15); // => 24

Years with Week 53

Some years have 53 ISO weeks (when January 1 is a Thursday, or a Wednesday in a leap year):

import { getISOWeek } from 'date-and-time';

getISOWeek(new Date(2020, 11, 31)); // => 53 (Dec 31, 2020)
getISOWeek(2020, 12, 31);          // => 53

See Also

• getISOWeekYear() - Get the ISO week year for a date
• week plugin - Format ISO week dates with format()