getLocaleEraNames

Signature

getLocaleEraNames(locale: string | string[], style?: LocaleNameStyle): string[]

Return a locale’s Gregorian era names as [BCE-label, CE-label].

The two-element array is [Before Common Era label, Common Era label].
Uses the host runtime’s Intl data via Intl.DateTimeFormat, so output depends on the runtime’s ICU build (full-ICU runtimes localize every locale; partial-ICU runtimes fall back to English).
If a locale has no distinct BCE/CE era names, both array elements contain the same string — the function never returns a sentinel for a valid locale, only for invalid input.
A -u-ca- subtag in locale is ignored. Both lookups pass calendar: "gregory" in the options bag, and ECMA-402’s ResolveLocale ranks an options value above the locale extension, so getLocaleEraNames("en-US-u-ca-buddhist") returns the same ["Before Christ", "Anno Domini"] as getLocaleEraNames("en-US"). That is deliberate — the function is Gregorian by contract — but it means the tag is accepted and overridden, not rejected. For another calendar’s names, format a date in it: formatDate("2024-03-15", "en-US-u-ca-buddhist", { era: "long", year: "numeric" }).
Returns [] if 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.

Parameter	Type	Description
`locale`	`string` \| `string[]`	BCP 47 locale tag (e.g. “en-US”, “fr-FR”, “ar-SA”), 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 []
`style`	`LocaleNameStyle`	Optional name style: `"long"` (default), `"short"`, or `"narrow"`

2-element [BCE-label, CE-label] array, or [] on invalid input

import { getLocaleEraNames } from "@northguild/gmt/plain/locale";

getLocaleEraNames("en-US") // ["Before Christ", "Anno Domini"]