feat: add function isValidTimezone

index.js

@@ -15,3 +15,4 @@ export { default as formatTimezone } from './formatTimezone.js'
 export { default as isSameDay } from './isSameDay.js'
 export * from './formatTimeOfDay.js'
 export * from './dhmsToDigits.js'
+export * from './isValidTimezone.js'

isValidTimezone.js

@@ -0,0 +1,26 @@
+/**
+ * Checks if a given time zone is valid.
+ *
+ * This function verifies whether a given time zone identifier is valid by
+ * attempting to create a DateTimeFormat object with the provided time zone.
+ *
+ * @param {string} tz - The time zone identifier to be validated.
+ * @returns {boolean} - Returns true if the time zone is valid, otherwise false.
+ * @throws {Error} - Throws an error if the environment does not support time zones.
+ *
+ * @example
+ * isValidTimeZone('America/New_York') // true
+ * isValidTimeZone('Invalid/Timezone') // false
+ */
+export function isValidTimezone (tz) {
+  if (!Intl || !Intl.DateTimeFormat().resolvedOptions().timeZone) {
+    throw new Error('Time zones are not available in this environment')
+  }
+
+  try {
+    Intl.DateTimeFormat(undefined, {timeZone: tz})
+    return true
+  } catch {
+    return false
+  }
+}

tests/isValidTimezone.test.js

@@ -0,0 +1,30 @@
+import { expect } from 'chai'
+import { isValidTimezone } from '../index.js'
+
+describe('isValidTimezone', () => {
+  it('should return true for a valid time zone', () => {
+    expect(isValidTimezone('America/New_York')).to.be.true
+  })
+
+  it('should return false for an invalid time zone', () => {
+    expect(isValidTimezone('Invalid/Timezone')).to.be.false
+  })
+
+  it('should return false for a valid GMT offset', () => {
+    expect(isValidTimezone('GMT+01:00')).to.be.false
+  })
+
+  it('should throw an error if time zones are not available in the environment', () => {
+    const originalIntl = global.Intl
+
+    // Mock the absence of Intl
+    global.Intl = undefined
+
+    try {
+      expect(() => isValidTimezone('America/New_York')).to.throw('Time zones are not available in this environment')
+    } finally {
+      // Restore the original Intl object
+      global.Intl = originalIntl
+    }
+  })
+})