new file mode 100644
--- /dev/null
+++ b/js/src/jit-test/tests/arguments/mapped-define.js
@@ -0,0 +1,20 @@
+function f(a) {
+    assertEq(arguments[0], 1);
+
+    Object.defineProperty(arguments, 0, {value: 23, writable: true, configurable: true});
+    assertEq(arguments[0], 23);
+    assertEq(a, 23);
+
+    a = 12;
+    assertEq(a, 12);
+    assertEq(arguments[0], 12);
+
+    Object.defineProperty(arguments, 0, {value: 9, writable: false, configurable: false});
+    assertEq(arguments[0], 9);
+    assertEq(a, 9);
+
+    a = 4;
+    assertEq(arguments[0], 9);
+    assertEq(a, 4);
+}
+f(1);

--- a/js/src/vm/ArgumentsObject.cpp
+++ b/js/src/vm/ArgumentsObject.cpp
@@ -646,24 +646,39 @@ MappedArgumentsObject::obj_definePropert
     bool isMapped = false;
     if (JSID_IS_INT(id)) {
         unsigned arg = unsigned(JSID_TO_INT(id));
         isMapped = arg < argsobj->initialLength() && !argsobj->isElementDeleted(arg);
     }
 
     // Step 4.
     Rooted<PropertyDescriptor> newArgDesc(cx, desc);
+
+    // Step 5.
     if (!desc.isAccessorDescriptor() && isMapped) {
-        // In this case the live mapping is supposed to keep working,
-        // we have to pass along the Getter/Setter otherwise they are overwritten.
-        newArgDesc.setGetter(MappedArgGetter);
-        newArgDesc.setSetter(MappedArgSetter);
+        // Step 5.a.
+        if (desc.hasWritable() && !desc.writable()) {
+            if (!desc.hasValue()) {
+                RootedValue v(cx, argsobj->element(JSID_TO_INT(id)));
+                newArgDesc.setValue(v);
+            }
+            newArgDesc.setGetter(nullptr);
+            newArgDesc.setSetter(nullptr);
+        } else {
+            // In this case the live mapping is supposed to keep working,
+            // we have to pass along the Getter/Setter otherwise they are
+            // overwritten.
+            newArgDesc.setGetter(MappedArgGetter);
+            newArgDesc.setSetter(MappedArgSetter);
+            newArgDesc.value().setUndefined();
+            newArgDesc.attributesRef() |= JSPROP_IGNORE_VALUE;
+        }
     }
 
-    // Steps 5-6. NativeDefineProperty will lookup [[Value]] for us.
+    // Step 6. NativeDefineProperty will lookup [[Value]] for us.
     if (!NativeDefineProperty(cx, obj.as<NativeObject>(), id, newArgDesc, result))
         return false;
     // Step 7.
     if (!result.ok())
         return true;
 
     // Step 8.
     if (isMapped) {