Get Start of Week by ISO Week Number

[taylorharwin]

How would I convert from ISO Week and Year numbers to a Date representing the start of a week? I see functions to do the reverse (getISOWeek). I'd like to go from this:

{week: 2, year: 2021}

to

"Week of January 11, 2021"

The options to do this on Stack Overflow all seemed very out of date 🥁

[fturmel]

Hi @taylorharwin,

You could solve this using parse, where the I token represents the ISO week of year and R represents the ISO week-numbering year:

import { parse } from "date-fns";

const date = parse("2 2021", "I R", new Date());
// Mon Jan 11 2021 00:00:00 GMT-0500 (Eastern Standard Time)

Here's an alternative, which I think would be lighter in terms of filesize comparatively to parse. It looks a bit clunky though, maybe there should be a new function for this that only takes in a year and an ISO week.

import { setISOWeek, startOfISOWeekYear } from "date-fns";

// careful! pass any date in the correct year to `startOfISOWeekYear`
// but not too close to Jan 1st because it might give you the start of the previous ISO year.
// Early january dates can be part of the last ISO week of the previous year.

const date = setISOWeek(startOfISOWeekYear(new Date(2021, 2, 1)), 2)
// Mon Jan 11 2021 00:00:00 GMT-0500 (Eastern Standard Time)

Then just format the date as needed:

format(date, "PPP");
// January 11th, 2021

format(date, 'MMMM d, yyyy')
// January 11, 2021

Hope this is helpful!