feat: add validateLanguageTag (#3)

src/index.ts

@@ -24,3 +24,20 @@ export function parseAcceptLanguage(value: string): string[] {
     !(tag === '*' || tag === '')
   )
 }
+
+/**
+ * validate the language tag whether is a well-formed {@link https://datatracker.ietf.org/doc/html/rfc4646#section-2.1 | BCP 47 language tag}.
+ *
+ * @param {string} lang a language tag
+ *
+ * @returns {boolean} Returns `true` if the language tag is valid, else `false`.
+ */
+export function validateLanguageTag(lang: string): boolean {
+  try {
+    // TODO: if we have a better way to validate the language tag, we should use it.
+    new Intl.Locale(lang)
+    return true
+  } catch {
+    return false
+  }
+}

test/index.test.ts

@@ -1,5 +1,9 @@
 import { describe, expect, test } from 'vitest'
-import { isLocale, parseAcceptLanguage } from '../src/index.ts'
+import {
+  isLocale,
+  parseAcceptLanguage,
+  validateLanguageTag,
+} from '../src/index.ts'
 
 describe('isLocale', () => {
   test('Locale instance', () => {
@@ -41,3 +45,13 @@ describe('parseAcceptLanguage', () => {
     expect(parseAcceptLanguage('')).toEqual([])
   })
 })
+
+describe('validateLanguageTag', () => {
+  test('valid', () => {
+    expect(validateLanguageTag('en-US')).toBe(true)
+  })
+
+  test('invalid', () => {
+    expect(validateLanguageTag('j')).toBe(false)
+  })
+})