From 2eb2bf3d39ca12c616a424a313658e61ddca7d0a Mon Sep 17 00:00:00 2001
From: Joel Hacke <Joel.Hacke@wwt.com>
Date: Fri, 1 Mar 2024 12:17:17 -0600
Subject: [PATCH] return null on toISOString if date is invalid

---
 src/index.js         | 2 +-
 test/display.test.js | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/index.js b/src/index.js
index 061ade178..291c0b041 100644
--- a/src/index.js
+++ b/src/index.js
@@ -420,7 +420,7 @@ class Dayjs {
     // ie 8 return
     // new Dayjs(this.valueOf() + this.$d.getTimezoneOffset() * 60000)
     // .format('YYYY-MM-DDTHH:mm:ss.SSS[Z]')
-    return this.$d.toISOString()
+    return this.isValid() ? this.$d.toISOString() : null
   }
 
   toString() {
diff --git a/test/display.test.js b/test/display.test.js
index ce1f47893..8d62d1953 100644
--- a/test/display.test.js
+++ b/test/display.test.js
@@ -261,6 +261,11 @@ it('As ISO 8601 String -> toISOString e.g. 2013-02-04T22:44:30.652Z', () => {
   expect(dayjs().toISOString()).toBe(moment().toISOString())
 })
 
+it('As null -> null', () => {
+  expect(dayjs(null).toISOString()).toBe(moment(null).toISOString())
+  expect(dayjs(null).toISOString()).toBe(null)
+})
+
 it('Year 1 formatted with YYYY should pad with zeroes', () => {
   const date = new Date(1, 0, 1)
   date.setUTCFullYear(1) // Required because 0-99 are parsed as 19xx in JS: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/Date#year