optional param to prevent function overwriting

src/utility.js

@@ -86,7 +86,23 @@ function sum(x) {
   return x.reduce((a, b) => a + b, 0);
 }
 
-function mergeInto(obj, other) {
+// options is optional input object containing mergeInto params
+// currently, it can contain
+//
+// key: noOverride, value: true
+// if it is set, it prevents symbol redefinition and shows error
+// in case of redefinition
+function mergeInto(obj, other, options = null) {
+  // check for unintended symbol redefinition
+  if (options && options.noOverride) {
+    for (const key of Object.keys(other)) {
+      if (obj.hasOwnProperty(key)) {
+        error('Symbol re-definition in JavaScript library: ' + key + '. Do not use noOverride if this is intended');
+        return;
+      }
+    }
+  }
+
   return Object.assign(obj, other);
 }
 

tests/test_other.py

@@ -3404,6 +3404,38 @@ def test_compilation_database(self):
     self.run_process([EMCC, 'a.c', '-MJ', 'hello.json', '-c', '-o', 'test.o'])
     self.assertContained('"file": "a.c", "output": "test.o"', read_file('hello.json'))
 
+  def test_duplicate_js_functions(self):
+    create_file('duplicated_func.c', '''
+      #include <stdio.h>
+      extern int duplicatedFunc();
+
+      int main() {
+        int res = duplicatedFunc();
+        printf("*%d*\\n", res);
+        return 0;
+      }
+    ''')
+    create_file('duplicated_func_1.js', '''
+      mergeInto(LibraryManager.library, {
+        duplicatedFunc : function() {
+            return 1;
+          }
+        }, { noOverride: true }
+      );
+    ''')
+    create_file('duplicated_func_2.js', '''
+      mergeInto(LibraryManager.library, {
+        duplicatedFunc : function() {
+            return 2;
+          }
+        }, { noOverride: true }
+      );
+    ''')
+
+    self.emcc_args += ['--js-library', 'duplicated_func_1.js', '--js-library', 'duplicated_func_2.js']
+    err = self.expect_fail([EMCC, 'duplicated_func.c'] + self.get_emcc_args())
+    self.assertContained('error: Symbol re-definition in JavaScript library: duplicatedFunc. Do not use noOverride if this is intended', err)
+
   def test_js_lib_quoted_key(self):
     create_file('lib.js', r'''
 mergeInto(LibraryManager.library, {