From e687ef9e0c64a9ab98195e98f05ea56b857c7949 Mon Sep 17 00:00:00 2001
From: dayongkr <dayong@tossbank.com>
Date: Sat, 24 Oct 2026 18:45:16 +0900
Subject: [PATCH] fix[startCase]: only uppercase the first letter of each word
 like lodash
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`startCase` lowercased the rest of every word that was not all uppercase. Since
#2097, `words` keeps camel humps in non-Latin scripts together like lodash, so
`startCase('ПриветМир')` turned into `'Приветмир'` instead of lodash's
`'ПриветМир'`. lodash only applies `upperFirst` to each word, and it does so by
code point. Do the same, so astral first letters such as `'𐐨'` are uppercased too.
---
 src/compat/string/startCase.spec.ts | 11 +++++++++++
 src/compat/string/startCase.ts      | 10 ++++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/src/compat/string/startCase.spec.ts b/src/compat/string/startCase.spec.ts
index 8f4aa839a..0afc0ea8d 100644
--- a/src/compat/string/startCase.spec.ts
+++ b/src/compat/string/startCase.spec.ts
@@ -66,4 +66,15 @@ describe('startCase', () => {
     expect(startCase('top10th')).toBe('Top 10th');
     expect(startCase('1st place 2nd 3rd 4th')).toBe('1st Place 2nd 3rd 4th');
   });
+
+  it('should only uppercase the first letter of each word', () => {
+    expect(startCase('ПриветМир')).toBe('ПриветМир');
+    expect(startCase('αβγΔεζ')).toBe('ΑβγΔεζ');
+    expect(startCase('fooBAR')).toBe('Foo BAR');
+    expect(startCase('mIxEd CaSe')).toBe('M Ix Ed Ca Se');
+  });
+
+  it('should uppercase a first letter outside the Basic Multilingual Plane', () => {
+    expect(startCase('𐐨𐐩 deseret')).toBe('𐐀 𐐁 Deseret');
+  });
 });
diff --git a/src/compat/string/startCase.ts b/src/compat/string/startCase.ts
index 0b8742480..f0c6679e6 100644
--- a/src/compat/string/startCase.ts
+++ b/src/compat/string/startCase.ts
@@ -27,11 +27,13 @@ export function startCase(str?: string): string {
       result += ' ';
     }
 
-    if (word === word.toUpperCase()) {
-      result += word;
-    } else {
-      result += word[0].toUpperCase() + word.slice(1).toLowerCase();
+    let firstLength = 1;
+
+    if (word.codePointAt(0)! > 0xffff) {
+      firstLength = 2;
     }
+
+    result += word.slice(0, firstLength).toUpperCase() + word.slice(firstLength);
   }
 
   return result;