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diff --git a/dom/bindings/JSSlots.h b/dom/bindings/JSSlots.h
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+++ b/dom/bindings/JSSlots.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/**
+ * This file defines various reserved slot indices used by JavaScript
+ * reflections of DOM objects.
+ */
+#ifndef mozilla_dom_DOMSlots_h
+#define mozilla_dom_DOMSlots_h
+
+// We use slot 0 for holding the raw object.  This is safe for both
+// globals and non-globals.
+// NOTE: This is baked into the Ion JIT as 0 in codegen for LGetDOMProperty and
+// LSetDOMProperty. Those constants need to be changed accordingly if this value
+// changes.
+#define DOM_OBJECT_SLOT 0
+
+// The total number of slots non-proxy DOM objects use by default.
+// Specific objects may have more for storing cached values.
+#define DOM_INSTANCE_RESERVED_SLOTS 1
+
+// Interface objects store a number of reserved slots equal to
+// DOM_INTERFACE_SLOTS_BASE + number of named constructors.
+#define DOM_INTERFACE_SLOTS_BASE 0
+
+// Interface prototype objects store a number of reserved slots equal to
+// DOM_INTERFACE_PROTO_SLOTS_BASE or DOM_INTERFACE_PROTO_SLOTS_BASE + 1 if a
+// slot for the unforgeable holder is needed.
+#define DOM_INTERFACE_PROTO_SLOTS_BASE 0
+
+// The slot index of raw pointer of dom object stored in observable array exotic
+// object. We need this in order to call the OnSet* and OnDelete* callbacks.
+#define OBSERVABLE_ARRAY_DOM_INTERFACE_SLOT 0
+
+// The slot index of backing list stored in observable array exotic object.
+#define OBSERVABLE_ARRAY_BACKING_LIST_OBJECT_SLOT 1
+
+#endif /* mozilla_dom_DOMSlots_h */