From 0245e40951780a7bce43ce9a0a97baa7e583f5c6 Mon Sep 17 00:00:00 2001
From: mori-hisayuki <37867997+mori-hisayuki@users.noreply.github.com>
Date: Thu, 7 Mar 2024 11:33:18 +0900
Subject: [PATCH] fix: unknown tz (#34)

* chore: add comment and type

* fix: This code ensures that if tz is undefined, the toLowerCase function is not called.
---
 src/deviceTZ.ts | 4 +++-
 src/format.ts   | 2 +-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/deviceTZ.ts b/src/deviceTZ.ts
index 3358cc7..56a6311 100644
--- a/src/deviceTZ.ts
+++ b/src/deviceTZ.ts
@@ -1,6 +1,8 @@
 /**
  * Get the timezone of the device.
+ *
+ * * Note: If the environment variable TZ is not set, it will return undefined.
  */
-export function deviceTZ() {
+export function deviceTZ(): string | undefined {
   return Intl.DateTimeFormat().resolvedOptions().timeZone
 }
diff --git a/src/format.ts b/src/format.ts
index f19f54c..475689a 100644
--- a/src/format.ts
+++ b/src/format.ts
@@ -77,7 +77,7 @@ export function format(
 
   // We need to apply an offset to the date so that it can be formatted as UTC.
   tz ??= deviceTZ()
-  if (tz.toLowerCase() !== "utc") {
+  if (tz?.toLowerCase() !== "utc") {
     inputDateOrOptions = removeOffset(
       inputDateOrOptions,
       offset(inputDateOrOptions, tz, "utc")