feat: support Intl.Locale instance checking (#1)

deno/mod.ts

@@ -0,0 +1 @@
+export * from '../src/index.ts'

src/index.ts

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+const objectToString = Object.prototype.toString
+const toTypeString = (value: unknown): string => objectToString.call(value)
+
+/**
+ * check whether the value is a {@link Intl.Locale} instance
+ *
+ * @param {unknown} val The locale value
+ *
+ * @returns {boolean} Returns `true` if the value is a {@link Intl.Locale} instance, else `false`.
+ */
+export function isLocale(val: unknown): val is Intl.Locale {
+  return toTypeString(val) === '[object Intl.Locale]'
+}

test/index.test.ts

@@ -0,0 +1,12 @@
+import { describe, expect, test } from 'vitest'
+import { isLocale } from '../src/index.ts'
+
+describe('isLocale', () => {
+  test('Locale instance', () => {
+    expect(isLocale(new Intl.Locale('en-US'))).toBe(true)
+  })
+
+  test('not Locale instance', () => {
+    expect(isLocale('en-US')).toBe(false)
+  })
+})