isoWeek() returns wrong value

[Daszczu]

Describe the bug
.isoWeek(week) should set proper week

Expected behavior
.isoWeek(week) sets week from previous year instead

Information

• Day.js Version v1.11.7
• OS: Windows
• Browser: Chrome 108
• Time zone: GMT +1

Reproduction code:

const dayjs = require("dayjs");
var isoWeek = require('dayjs/plugin/isoWeek')
dayjs.extend(isoWeek);

year = 2021;
week = 51;

date = dayjs();
date2021 = dayjs().year(year);
date2021week51 = dayjs().year(year).isoWeek(week);
    
console.log(date); // correct
console.log(date2021); // correct 
console.log(date2021week51); // incorrect - should be week 51 of year 2021, not 2020

https://runkit.com/63b3f5320465cd00088b7bd9/63b3f5e70465cd00088b7c55

[KronosDev-Pro]

Week 51 => Monday 20 December 2021 - Sunday 26 December 2021

Information

• Day.js v1.11.7
• OS: Windows 11
• Node: v18.7.0

const dayjs = require("dayjs")
const isoWeek = require('dayjs/plugin/isoWeek')

dayjs.extend(isoWeek)

const date = dayjs(new Date(2021, 0, 10, 10, 48, 35))
const date2021week51 = date.isoWeek(51)

console.log(date) // correct, '2021-01-10T09:48:35.000Z'
console.log(date2021week51) // correct, '2021-12-26T09:48:35.000Z'

I'll try it on a browser after lunch 😉

[KronosDev-Pro]

Information

• Day.js v1.11.7
• OS: Windows 11
• Browser: Brave v108 (Chromium v108)

<script src="https://cdn.jsdelivr.net/npm/dayjs@1/dayjs.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/dayjs@1/plugin/isoWeek.js"></script>
<script>
    dayjs.extend(window.dayjs_plugin_isoWeek);
    console.log(dayjs().format(), dayjs().isoWeek()); // test dayjs & plugin loaded correctly
    const date = dayjs(new Date(2021, 0, 10, 10, 48, 35))
    const date2021week51 = date.isoWeek(51)

    console.log(date.format()) // correct, '2021-01-10T10:48:35+01:00'
    console.log(date2021week51.format()) // correct, '2021-12-26T10:48:35+01:00'
</script>

<script src="https://cdn.jsdelivr.net/npm/dayjs@1/dayjs.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/dayjs@1/plugin/isoWeek.js"></script>
<script>
    dayjs.extend(window.dayjs_plugin_isoWeek);
    console.log(dayjs().format(), dayjs().isoWeek()); // test dayjs & plugin loaded correctly

    const year = 2021;
    const week = 51;

    date = dayjs();
    const date2021 = dayjs().year(year);
    date2021week51 = dayjs().year(year).isoWeek(week);

    console.log(date.format()); // correct, '2023-01-10T15:40:55+01:00'
    console.log(date2021.format()); // correct, '2021-01-10T15:40:55+01:00'
    console.log(date2021week51.format()); // correct, '2021-12-26T15:40:55+01:00'
</script>

I can't reproduce your error !!!

[Daszczu]

It was wrong the day I posted it

The link for runkit presents the behavior

when you run the same code today - it works fine

maybe it was related to the fact, that the dajys() returns current date which was then close to the beginning of the year and somehow iso week was calculated wrongly thinking it's still the previous year?
that I don't know, but I could observe this isssue during that day on multiple devices and platforms

[20manas]

This is covered here: #1332

When you ran your code (around 1-2 Jan), the week number belonged to the previous year, which is why setting the week number was changing the year. But a few days later (from 3 Jan onwards), the week started belonging to 2023, so the code's behavior changed.

The "buggy" behavior can be reproduced like this:

Of course this behavior isn't really buggy, but the result of the definition of the ISO week.

One way to resolve this is to make sure that the date is set to at least 4th of January before setting the week number since the first week of any year will always contain 4th of January. So something like this:

dayjs({year: 2023, day: 4}).isoWeek(week).day(1);
// the year will be 2023 as long as week is between 1 - 52 and the day of the week will be set as Monday

Perhaps this should be documented somewhere as this is an easy mistake to make.