Well, the title is pretty explanatory - I need to figure out the day week start in local - it can be Monday, Sunday, Saturday or Friday - in pure Javascript.

I found this

<firstDay day="mon" territories="001 AD AI AL AM AN AT AX AZ BA BE BG BM BN BY CH CL CM CR CY CZ DE DK EC EE ES FI FJ FO FR GB GE GF GP GR HR HU IS IT KG KZ LB LI LK LT LU LV MC MD ME MK MN MQ MY NL NO PL PT RE RO RS RU SE SI SK SM TJ TM TR UA UY UZ VA VN XK" />
<firstDay day="fri" territories="BD MV" />
<firstDay day="sat" territories="AE AF BH DJ DZ EG IQ IR JO KW LY MA OM QA SD SY" />
<firstDay day="sun" territories="AG AR AS AU BR BS BT BW BZ CA CN CO DM DO ET GT GU HK HN ID IE IL IN JM JP KE KH KR LA MH MM MO MT MX MZ NI NP NZ PA PE PH PK PR PY SA SG SV TH TN TT TW UM US VE VI WS YE ZA ZW" />
<firstDay day="sun" territories="GB" alt="variant" references="Shorter Oxford Dictionary (5th edition, 2002)" />

found a compatible table of ISO3166 and ISO639 codes -

Well, the title is pretty explanatory - I need to figure out the day week start in local - it can be Monday, Sunday, Saturday or Friday - in pure Javascript.

I found this https://stackoverflow.com/a/727536/1226226

<firstDay day="mon" territories="001 AD AI AL AM AN AT AX AZ BA BE BG BM BN BY CH CL CM CR CY CZ DE DK EC EE ES FI FJ FO FR GB GE GF GP GR HR HU IS IT KG KZ LB LI LK LT LU LV MC MD ME MK MN MQ MY NL NO PL PT RE RO RS RU SE SI SK SM TJ TM TR UA UY UZ VA VN XK" />
<firstDay day="fri" territories="BD MV" />
<firstDay day="sat" territories="AE AF BH DJ DZ EG IQ IR JO KW LY MA OM QA SD SY" />
<firstDay day="sun" territories="AG AR AS AU BR BS BT BW BZ CA CN CO DM DO ET GT GU HK HN ID IE IL IN JM JP KE KH KR LA MH MM MO MT MX MZ NI NP NZ PA PE PH PK PR PY SA SG SV TH TN TT TW UM US VE VI WS YE ZA ZW" />
<firstDay day="sun" territories="GB" alt="variant" references="Shorter Oxford Dictionary (5th edition, 2002)" />

found a compatible table of ISO3166 and ISO639 codes - https://wiki.openstreetmap.org/wiki/Nominatim/Country_Codes

• javascript
• date
• weekday

• 1 stackoverflow.com/questions/5210376/… – user6702203 Commented Nov 19, 2018 at 21:01
• Possible duplicate of How to get first and last day of the week in JavaScript – hindmost Commented Nov 19, 2018 at 21:08
• @hindmost - nope, this code shows that the first day of the week is Sunday, but in my current locale it should be Monday – Oleh Melnyk Commented Nov 19, 2018 at 21:19
• Fitting that answer to your specific need is trivial. – hindmost Commented Nov 19, 2018 at 21:23
• 6 @hindmost - not so trivial to me - there are countries where weeks start from Monday, Sunday, Saturday or even Friday – Oleh Melnyk Commented Nov 19, 2018 at 21:26

5 Answers 5

Without moment. Essentially a heavily golfed version of https://github.com/gamtiq/weekstart, ignoring some unusual locales. No pure JS answer (using APIs available as of 2020) is going to be perfect, but this will yield the same result as a system API would for at least 99.9% of users.

https://github.com/tc39/proposal-intl-locale-info has been proposed which will include an API level solution for this.

function weekStart(region, language) {
  const regionSat = 'AEAFBHDJDZEGIQIRJOKWLYOMQASDSY'.match(/../g);
  const regionSun = 'AGARASAUBDBRBSBTBWBZCACNCODMDOETGTGUHKHNIDILINJMJPKEKHKRLAMHMMMOMTMXMZNINPPAPEPHPKPRPTPYSASGSVTHTTTWUMUSVEVIWSYEZAZW'.match(/../g);
  const languageSat = ['ar','arq','arz','fa'];
  const languageSun = 'amasbndzengnguhehiidjajvkmknkolomhmlmrmtmyneomorpapssdsmsnsutatethtnurzhzu'.match(/../g);

  return (
    region ? (
      regionSun.includes(region) ? 'sun' :
      regionSat.includes(region) ? 'sat' : 'mon') : (
      languageSun.includes(language) ? 'sun' :
      languageSat.includes(language) ? 'sat' : 'mon'));
}

function weekStartLocale(locale) {
  const parts = locale.match(/^([a-z]{2,3})(?:-([a-z]{3})(?=$|-))?(?:-([a-z]{4})(?=$|-))?(?:-([a-z]{2}|\d{3})(?=$|-))?/i);
  return weekStart(parts[4], parts[1]);
}

console.log(weekStartLocale(navigator.language));
console.log(weekStartLocale('en'));
console.log(weekStartLocale('en-GB'));
console.log(weekStartLocale('ar-AE'));

As of lately you can use: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Locale/weekInfo

It is available in all browsers (except firefox: March 2023)

code looks like this:

new Intl.Locale(navigator.language).weekInfo.firstDay // 1 or 7 ...

you can check for any locale by changing navigator.language with the desired language ex:

new Intl.Locale('fr').weekInfo.firstDay // 1

A more maintainable and usable version based on CLDR.

Based on country code. Returns 0 for Sunday, 1 for Monday, -1 for Saturday and -2 for Friday so you can use it to make a calendar. Returns 1 for unknown country codes.

{
let wso = { // fri:1, sat:2, sun:3
    MV:1,
    AE:2,AF:2,BH:2,DJ:2,DZ:2,EG:2,IQ:2,IR:2,JO:2,KW:2,LY:2,OM:2,QA:2,SD:2,SY:2,
    AG:3,AS:3,AU:3,BD:3,BR:3,BS:3,BT:3,BW:3,BZ:3,CA:3,CN:3,CO:3,DM:3,DO:3,ET:3,
    GT:3,GU:3,HK:3,HN:3,ID:3,IL:3,IN:3,JM:3,JP:3,KE:3,KH:3,KR:3,LA:3,MH:3,MM:3,
    MO:3,MT:3,MX:3,MZ:3,NI:3,NP:3,PA:3,PE:3,PH:3,PK:3,PR:3,PT:3,PY:3,SA:3,SG:3,
    SV:3,TH:3,TT:3,TW:3,UM:3,US:3,VE:3,VI:3,WS:3,YE:3,ZA:3,ZW:3,
}
function week_start_offset(country) {
    return (wso[country] || 4) - 3
}
}

console.log(week_start_offset('US'))
console.log(week_start_offset('RO'))

Following on from @user1928596

Their suggestion almost worked, but because my browser language is different to my date settings. I ended up using this.

  // Default to en-GB which is Monday -> Sunday
  const dateLocale = Intl
    ? Intl?.DateTimeFormat()?.resolvedOptions()?.locale
    : "en-GB"

  // @ts-ignore .weekInfo exists on all browsers except Firefox.
  const weekFirstDay = new Intl.Locale(dateLocale)?.weekInfo?.firstDay ?? 1

you could use moment.js

moment.localeData('en-us').firstDayOfWeek();

As for pure js, you would have to make your own lookup table. You can use momentjs source code for reference. https://github.com/moment/moment/tree/develop/locale

In each of each locale file look for the dow at the end of the config.