From 0e605ae26e70fc2efdef31178c77d676c8625f28 Mon Sep 17 00:00:00 2001
From: Brendan Dahl <brendan.dahl@gmail.com>
Date: Wed, 7 Jan 2026 19:53:26 +0000
Subject: [PATCH 1/2] Fix named JS classes in libraries.

When a JS library contains a named class, the previous logic blindly prepended the mangled name, resulting in invalid syntax like 'class  MyClass MyClass'.
This change detects if a class name is already present (excluding 'extends') and replaces it correctly.

Fixes #25840
---
 src/Fetch.js       |  3 +--
 src/jsifier.mjs    |  2 +-
 test/test_jslib.py | 28 ++++++++++++++++++++++++++++
 3 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/src/Fetch.js b/src/Fetch.js
index 52619b454e936..40897bd7b996d 100644
--- a/src/Fetch.js
+++ b/src/Fetch.js
@@ -10,8 +10,7 @@
  * This implementation is specifically tailored to only handle 'arraybuffer'
  * responses.
  */
-// TODO Use a regular class name when #5840 is fixed.
-var FetchXHR = class {
+class FetchXHR {
   // --- Public XHR Properties ---
 
   // Event Handlers
diff --git a/src/jsifier.mjs b/src/jsifier.mjs
index 0d9f9aeddffa6..c28d81aae16f2 100644
--- a/src/jsifier.mjs
+++ b/src/jsifier.mjs
@@ -717,7 +717,7 @@ function(${args}) {
           contentText = `var ${mangled} = ` + contentText + ';';
         } else if (contentText.startsWith('class ')) {
           // Handle class declarations (which also have typeof == 'function'.)
-          contentText = contentText.replace(/^class /, `class ${mangled} `);
+          contentText = contentText.replace(/^class(?:\s+(?!extends\b)[^{\s]+)?/, `class ${mangled}`);
         } else {
           // Handle regular (non-arrow) functions
           contentText = contentText.replace(/function(?:\s+([^(]+))?\s*\(/, `function ${mangled}(`);
diff --git a/test/test_jslib.py b/test/test_jslib.py
index a2d8b063c9bb8..7c07d975151ab 100644
--- a/test/test_jslib.py
+++ b/test/test_jslib.py
@@ -719,3 +719,31 @@ def test_jslib_version_check(self):
       #endif
     ''')
     self.assert_fail([EMCC, '--js-library=libfoo.js'], 'error: libfoo.js:3: #error "library does not support emscripten > 3.0.0"')
+
+  def test_jslib_named_class(self):
+    create_file('lib.js', r'''
+      class MyClass {
+        constructor() { this.x = 42; }
+      }
+
+      addToLibrary({
+        $MyClass: MyClass,
+        log_class__deps: ['$MyClass'],
+        log_class: () => {
+          var i = new MyClass();
+          out('MyClass: ' + i.x);
+        }
+      });
+    ''')
+    create_file('src.c', r'''
+      #include <stdio.h>
+      #include <emscripten.h>
+
+      extern void log_class();
+
+      int main() {
+        log_class();
+        return 0;
+      }
+    ''')
+    self.do_runf('src.c', 'MyClass: 42', cflags=['--js-library', 'lib.js'])

From 7f8e22a37f92d1bf35ebdb83031267ae806b6794 Mon Sep 17 00:00:00 2001
From: Brendan Dahl <brendan.dahl@gmail.com>
Date: Thu, 8 Jan 2026 01:29:13 +0000
Subject: [PATCH 2/2] comment

---
 test/test_jslib.py | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/test/test_jslib.py b/test/test_jslib.py
index 7c07d975151ab..266408a824eb3 100644
--- a/test/test_jslib.py
+++ b/test/test_jslib.py
@@ -722,12 +722,13 @@ def test_jslib_version_check(self):
 
   def test_jslib_named_class(self):
     create_file('lib.js', r'''
-      class MyClass {
-        constructor() { this.x = 42; }
-      }
-
+      class ParentClass {}
       addToLibrary({
-        $MyClass: MyClass,
+        $ParentClass: ParentClass,
+        $MyClass__deps: ['$ParentClass'],
+        $MyClass: class extends ParentClass {
+          constructor() { super(); this.x = 42; }
+        },
         log_class__deps: ['$MyClass'],
         log_class: () => {
           var i = new MyClass();