getLocaleDayOfWeek

Signature

getLocaleDayOfWeek(value: string, locale: string | string[]): number | null

Return the locale-relative day-of-week index for value.

• 0 = the locale’s first day of week (e.g. Sunday for en-US, Monday for fr-FR).
• Resolves the locale’s first day of week via Intl.Locale.prototype.weekInfo.
• Falls back to Monday if the runtime’s weekInfo data doesn’t resolve a first day for the locale.
• Returns null if value is not a valid ISO date or locale is not a well-formed BCP 47 tag (ECMA-402 IsWellFormedLanguageTag). A well-formed tag with no matching locale data is not an error: it falls back to the host’s default locale, as ECMA-402 ResolveLocale requires.

Parameters

Parameter	Type	Description
value	string	ISO 8601 date string
locale	string | string[]	BCP 47 locale tag (e.g. “en-US”, “fr-FR”), or a preference list of tags (ECMA-402; the first with locale data is read). Required: omitted, or an empty list (which ECMA-402 would resolve to the host default), returns null

Returns

locale-relative day-of-week index (0–6) or null on invalid input

Examples

import { getLocaleDayOfWeek } from "@northguild/gmt/plain/calculate";

getLocaleDayOfWeek("2024-02-25", "en-US") // 0 (Sunday)

valuelocale

output

 

Source

plain/calculate/getLocaleDayOfWeek.ts