Configring node js.

node_modules/core-js/modules/_keyof.js

@@ -0,0 +1,10 @@
+var getKeys   = require('./_object-keys')
+  , toIObject = require('./_to-iobject');
+module.exports = function(object, el){
+  var O      = toIObject(object)
+    , keys   = getKeys(O)
+    , length = keys.length
+    , index  = 0
+    , key;
+  while(length > index)if(O[key = keys[index++]] === el)return key;
+};