Fix named JS classes in libraries.

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

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}(`);

test/test_jslib.py

@@ -719,3 +719,32 @@ 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 ParentClass {}
+      addToLibrary({
+        $ParentClass: ParentClass,
+        $MyClass__deps: ['$ParentClass'],
+        $MyClass: class extends ParentClass {
+          constructor() { super(); this.x = 42; }
+        },
+        log_class__deps: ['$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'])