From e3fb33c473e493867f0d5e3ed1b68b85c7257aa8 Mon Sep 17 00:00:00 2001
From: camgraff <46980357+camgraff@users.noreply.github.com>
Date: Sun, 13 Sep 2020 05:56:40 -0500
Subject: [PATCH] [bugfix] Fix bug where .format() modifies original moment
 instance (#5580)

* Made era functions clone moment instance before calling startOf()

* Add regression tests

* Run prettier-fmt
---
 src/lib/units/era.js      |  8 ++++----
 src/test/moment/format.js | 22 ++++++++++++++++++++++
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/src/lib/units/era.js b/src/lib/units/era.js
index 28c71495d..805814619 100644
--- a/src/lib/units/era.js
+++ b/src/lib/units/era.js
@@ -144,7 +144,7 @@ export function getEraName() {
         eras = this.localeData().eras();
     for (i = 0, l = eras.length; i < l; ++i) {
         // truncate time
-        val = this.startOf('day').valueOf();
+        val = this.clone().startOf('day').valueOf();
 
         if (eras[i].since <= val && val <= eras[i].until) {
             return eras[i].name;
@@ -164,7 +164,7 @@ export function getEraNarrow() {
         eras = this.localeData().eras();
     for (i = 0, l = eras.length; i < l; ++i) {
         // truncate time
-        val = this.startOf('day').valueOf();
+        val = this.clone().startOf('day').valueOf();
 
         if (eras[i].since <= val && val <= eras[i].until) {
             return eras[i].narrow;
@@ -184,7 +184,7 @@ export function getEraAbbr() {
         eras = this.localeData().eras();
     for (i = 0, l = eras.length; i < l; ++i) {
         // truncate time
-        val = this.startOf('day').valueOf();
+        val = this.clone().startOf('day').valueOf();
 
         if (eras[i].since <= val && val <= eras[i].until) {
             return eras[i].abbr;
@@ -207,7 +207,7 @@ export function getEraYear() {
         dir = eras[i].since <= eras[i].until ? +1 : -1;
 
         // truncate time
-        val = this.startOf('day').valueOf();
+        val = this.clone().startOf('day').valueOf();
 
         if (
             (eras[i].since <= val && val <= eras[i].until) ||
diff --git a/src/test/moment/format.js b/src/test/moment/format.js
index 1ea1e9f0e..7fbcafc01 100644
--- a/src/test/moment/format.js
+++ b/src/test/moment/format.js
@@ -927,3 +927,25 @@ test('HTML5_FMT.WEEK', function (assert) {
         'issue #4833 regression'
     );
 });
+
+test('does not modify original moment instance', function (assert) {
+    let m = moment(0),
+        original = moment(0);
+    m.format('yyyy-MM-DD');
+    assert.equal(
+        m.toISOString(),
+        original.toISOString(),
+        'issue #5571 regression'
+    );
+
+    assert.deepEqual(
+        moment(new Date('2020-08-11 23:59:59'))
+            .format('yyyy-MM-DD HH:mm:ss')
+            .split(' '),
+        moment(new Date('2020-08-11 23:59:59'))
+            .format('HH:mm:ss yyyy-MM-DD')
+            .split(' ')
+            .reverse(),
+        'issue #5681 regression'
+    );
+});
-- 
2.47.3