--- a/js/src/builtin/intl/ListFormat.cpp
+++ b/js/src/builtin/intl/ListFormat.cpp
@@ -245,24 +245,27 @@ static bool FormatListToParts(JSContext*
                               MutableHandleValue result) {
   mozilla::intl::ListFormat::PartVector parts;
   auto formatResult = lf->FormatToParts(list, parts);
   if (formatResult.isErr()) {
     js::intl::ReportInternalError(cx, formatResult.unwrapErr());
     return false;
   }
 
-  RootedArrayObject partsArray(cx, NewDenseEmptyArray(cx));
+  RootedArrayObject partsArray(cx,
+                               NewDenseFullyAllocatedArray(cx, parts.length()));
   if (!partsArray) {
     return false;
   }
+  partsArray->ensureDenseInitializedLength(0, parts.length());
 
   RootedObject singlePart(cx);
   RootedValue val(cx);
 
+  size_t index = 0;
   for (const mozilla::intl::ListFormat::Part& part : parts) {
     singlePart = NewPlainObject(cx);
     if (!singlePart) {
       return false;
     }
 
     if (part.first == mozilla::intl::ListFormat::PartType::Element) {
       val = StringValue(cx->names().element);
@@ -279,20 +282,19 @@ static bool FormatListToParts(JSContext*
     if (!partStr) {
       return false;
     }
     val = StringValue(partStr);
     if (!DefineDataProperty(cx, singlePart, cx->names().value, val)) {
       return false;
     }
 
-    if (!NewbornArrayPush(cx, partsArray, ObjectValue(*singlePart))) {
-      return false;
-    }
+    partsArray->initDenseElement(index++, ObjectValue(*singlePart));
   }
+  MOZ_ASSERT(index == parts.length());
 
   result.setObject(*partsArray);
   return true;
 }
 
 bool js::intl_FormatList(JSContext* cx, unsigned argc, Value* vp) {
   CallArgs args = CallArgsFromVp(argc, vp);
   MOZ_ASSERT(args.length() == 3);