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diff --git a/dom/bindings/ObservableArrayProxyHandler.h b/dom/bindings/ObservableArrayProxyHandler.h
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+++ b/dom/bindings/ObservableArrayProxyHandler.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/. */
+
+#ifndef mozilla_dom_ObservableArrayProxyHandler_h
+#define mozilla_dom_ObservableArrayProxyHandler_h
+
+#include "js/TypeDecls.h"
+#include "js/Wrapper.h"
+
+namespace mozilla::dom {
+
+/**
+ * Proxy handler for observable array exotic object.
+ *
+ * The indexed properties are stored in the backing list object in reserved slot
+ * of the proxy object with special treatment intact.
+ *
+ * The additional properties are stored in the proxy target object.
+ */
+
+class ObservableArrayProxyHandler : public js::ForwardingProxyHandler {
+ public:
+  explicit constexpr ObservableArrayProxyHandler()
+      : js::ForwardingProxyHandler(&family) {}
+
+  // Implementations of methods that can be implemented in terms of
+  // other lower-level methods.
+  bool defineProperty(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                      JS::Handle<JS::PropertyKey> aId,
+                      JS::Handle<JS::PropertyDescriptor> aDesc,
+                      JS::ObjectOpResult& aResult) const override;
+
+  bool delete_(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+               JS::Handle<JS::PropertyKey> aId,
+               JS::ObjectOpResult& aResult) const override;
+
+  bool get(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+           JS::Handle<JS::Value> aReceiver, JS::Handle<JS::PropertyKey> aId,
+           JS::MutableHandle<JS::Value> aVp) const override;
+
+  bool getOwnPropertyDescriptor(
+      JSContext* aCx, JS::Handle<JSObject*> aProxy,
+      JS::Handle<JS::PropertyKey> aId,
+      JS::MutableHandle<Maybe<JS::PropertyDescriptor>> aDesc) const override;
+
+  bool has(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+           JS::Handle<JS::PropertyKey> aId, bool* aBp) const override;
+
+  bool ownPropertyKeys(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                       JS::MutableHandleVector<jsid> aProps) const override;
+
+  bool preventExtensions(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                         JS::ObjectOpResult& aResult) const override;
+
+  bool set(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+           JS::Handle<JS::PropertyKey> aId, JS::Handle<JS::Value> aV,
+           JS::Handle<JS::Value> aReceiver,
+           JS::ObjectOpResult& aResult) const override;
+
+  bool SetLength(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                 uint32_t aLength) const;
+
+  static const char family;
+
+ protected:
+  bool GetBackingListObject(
+      JSContext* aCx, JS::Handle<JSObject*> aProxy,
+      JS::MutableHandle<JSObject*> aBackingListObject) const;
+
+  bool GetBackingListLength(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                            uint32_t* aLength) const;
+
+  bool SetLength(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                 JS::Handle<JSObject*> aBackingList, uint32_t aLength,
+                 JS::ObjectOpResult& aResult) const;
+
+  bool SetLength(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                 JS::Handle<JSObject*> aBackingList,
+                 JS::Handle<JS::Value> aValue,
+                 JS::ObjectOpResult& aResult) const;
+
+  // Hook for subclasses to invoke the setting the indexed value steps which
+  // would invoke DeleteAlgorithm/SetAlgorithm defined and implemented per
+  // interface. Returns false and throw exception on failure.
+  virtual bool SetIndexedValue(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                               JS::Handle<JSObject*> aBackingList,
+                               uint32_t aIndex, JS::Handle<JS::Value> aValue,
+                               JS::ObjectOpResult& aResult) const = 0;
+
+  // Hook for subclasses to invoke the DeleteAlgorithm defined and implemented
+  // per interface. Returns false and throw exception on failure.
+  virtual bool OnDeleteItem(JSContext* aCx, JS::Handle<JSObject*> aProxy,
+                            JS::Handle<JS::Value> aValue,
+                            uint32_t aIndex) const = 0;
+};
+
+inline bool IsObservableArrayProxy(JSObject* obj) {
+  return js::IsProxy(obj) && js::GetProxyHandler(obj)->family() ==
+                                 &ObservableArrayProxyHandler::family;
+}
+
+inline const ObservableArrayProxyHandler* GetObservableArrayProxyHandler(
+    JSObject* obj) {
+  MOZ_ASSERT(IsObservableArrayProxy(obj));
+  return static_cast<const ObservableArrayProxyHandler*>(
+      js::GetProxyHandler(obj));
+}
+
+}  // namespace mozilla::dom
+
+#endif /* mozilla_dom_ObservableArrayProxyHandler_h */