I’m currently building a calendar data display for Atuin, and wanted to ensure that the week started with the correct day.

While I’m a big fan of things being configurable and flexible, I don’t want to introduce a new config option unless it’s required!

Luckily browser localisation exposes plenty of options.

Get a Locale object

First we need a Locale! They’re created like so

new Intl.Locale('CODE')

// eg

new Intl.Locale('en-GB')

We can get the code as follows

new Intl.Locale(navigator.language)

This is a set by the user in their browser

Get the weekinfo

Once we have the Locale, it’s simple

let locale = new Intl.Locale(navigator.language);
let weekinfo = locale.getWeekInfo();
console.log(weekinfo.firstDay); // prints 1 for me, in the UK

Docs here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Locale/getWeekInfo