--- a/content/events/src/nsDOMEvent.cpp
+++ b/content/events/src/nsDOMEvent.cpp
@@ -56,16 +56,17 @@
 #include "nsGkAtoms.h"
 #include "nsMutationEvent.h"
 #include "nsContentUtils.h"
 #include "nsIURI.h"
 #include "nsIScriptSecurityManager.h"
 #include "nsIScriptError.h"
 #include "nsDOMPopStateEvent.h"
 #include "mozilla/Preferences.h"
+#include "nsJSUtils.h"
 
 using namespace mozilla;
 
 static const char* const sEventNames[] = {
   "mousedown", "mouseup", "click", "dblclick", "mouseenter", "mouseleave", "mouseover",
   "mouseout", "MozMouseHittest", "mousemove", "contextmenu", "keydown", "keyup", "keypress",
   "focus", "blur", "load", "popstate", "beforescriptexecute",
   "afterscriptexecute", "beforeunload", "unload",
@@ -400,25 +401,19 @@ nsDOMEvent::Initialize(nsISupports* aOwn
 
   JSAutoRequest ar(aCx);
   JSString* jsstr = JS_ValueToString(aCx, aArgv[0]);
   if (!jsstr) {
     return NS_ERROR_DOM_SYNTAX_ERR;
   }
 
   JS::Anchor<JSString*> deleteProtector(jsstr);
-  size_t length;
-  const jschar* chars = JS_GetStringCharsAndLength(aCx, jsstr, &length);
-  if (!chars) {
-    return NS_ERROR_OUT_OF_MEMORY;
-  }
 
-  nsAutoString type;
-  type.Assign(chars, length);
-  deleteProtector.clear();
+  nsDependentJSString type;
+  NS_ENSURE_STATE(type.init(aCx, jsstr));
   
   nsCOMPtr<nsISupports> dict;
   if (aArgc >= 2 && !JSVAL_IS_PRIMITIVE(aArgv[1])) {
     nsContentUtils::XPConnect()->WrapJS(aCx, JSVAL_TO_OBJECT(aArgv[1]),
                                         EventInitIID(),
                                         getter_AddRefs(dict));
   }
   nsresult rv = InitFromCtor(type, dict);