diff --git a/js/src/builtin/MapObject.cpp b/js/src/builtin/MapObject.cpp
index 8b7d4f4c6bb7..05fc32d14ff5 100644
--- a/js/src/builtin/MapObject.cpp
+++ b/js/src/builtin/MapObject.cpp
@@ -1237,11 +1237,24 @@ MapObject::construct(JSContext *cx, unsigned argc, Value *vp)
 
     CallArgs args = CallArgsFromVp(argc, vp);
     if (!args.get(0).isNullOrUndefined()) {
+        RootedValue adderVal(cx);
+        if (!JSObject::getProperty(cx, obj, obj, cx->names().set, &adderVal))
+            return false;
+
+        if (!IsCallable(adderVal))
+            return ReportIsNotFunction(cx, adderVal);
+
+        bool isOriginalAdder = IsNativeFunction(adderVal, MapObject::set);
+        RootedValue mapVal(cx, ObjectValue(*obj));
+        FastInvokeGuard fig(cx, adderVal);
+        InvokeArgs &args2 = fig.args();
+
         ForOfIterator iter(cx);
         if (!iter.init(args[0]))
             return false;
         RootedValue pairVal(cx);
         RootedObject pairObj(cx);
+        AutoHashableValueRooter hkey(cx);
         ValueMap *map = obj->getData();
         while (true) {
             bool done;
@@ -1263,20 +1276,32 @@ MapObject::construct(JSContext *cx, unsigned argc, Value *vp)
             if (!JSObject::getElement(cx, pairObj, pairObj, 0, &key))
                 return false;
 
-            AutoHashableValueRooter hkey(cx);
-            if (!hkey.setValue(cx, key))
-                return false;
-
             RootedValue val(cx);
             if (!JSObject::getElement(cx, pairObj, pairObj, 1, &val))
                 return false;
 
-            RelocatableValue rval(val);
-            if (!map->put(hkey, rval)) {
-                js_ReportOutOfMemory(cx);
-                return false;
+            if (isOriginalAdder) {
+                if (!hkey.setValue(cx, key))
+                    return false;
+
+                RelocatableValue rval(val);
+                if (!map->put(hkey, rval)) {
+                    js_ReportOutOfMemory(cx);
+                    return false;
+                }
+                WriteBarrierPost(cx->runtime(), map, key);
+            } else {
+                if (!args2.init(2))
+                    return false;
+
+                args2.setCallee(adderVal);
+                args2.setThis(mapVal);
+                args2[0].set(key);
+                args2[1].set(val);
+
+                if (!fig.invoke(cx))
+                    return false;
             }
-            WriteBarrierPost(cx->runtime(), map, key);
         }
     }