diff -r 000000000000 -r 6474c204b198 dom/base/nsDOMJSUtils.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/base/nsDOMJSUtils.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,46 @@
+/* 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 nsDOMJSUtils_h__
+#define nsDOMJSUtils_h__
+
+#include "nsIScriptContext.h"
+#include "jsapi.h"
+
+class nsIJSArgArray;
+
+// seems like overkill for just this 1 function - but let's see what else
+// falls out first.
+inline nsIScriptContext *
+GetScriptContextFromJSContext(JSContext *cx)
+{
+  if (!(JS::ContextOptionsRef(cx).privateIsNSISupports())) {
+    return nullptr;
+  }
+
+  nsCOMPtr<nsIScriptContext> scx =
+    do_QueryInterface(static_cast<nsISupports *>
+                                 (::JS_GetContextPrivate(cx)));
+
+  // This will return a pointer to something that's about to be
+  // released, but that's ok here.
+  return scx;
+}
+
+JSObject* GetDefaultScopeFromJSContext(JSContext *cx);
+
+// A factory function for turning a JS::Value argv into an nsIArray
+// but also supports an effecient way of extracting the original argv.
+// Bug 312003 describes why this must be "void *", but argv will be cast to
+// JS::Value* and the args are found at:
+//    ((JS::Value*)aArgv)[0], ..., ((JS::Value*)aArgv)[aArgc - 1]
+// The resulting object will take a copy of the array, and ensure each
+// element is rooted.
+// Optionally, aArgv may be nullptr, in which case the array is allocated and
+// rooted, but all items remain nullptr.  This presumably means the caller
+// will then QI us for nsIJSArgArray, and set our array elements.
+nsresult NS_CreateJSArgv(JSContext *aContext, uint32_t aArgc, void *aArgv,
+                         nsIJSArgArray **aArray);
+
+#endif // nsDOMJSUtils_h__