diff --git a/modules/EchofonSign.jsm b/modules/EchofonSign.jsm
index 22a5d38..80664ed 100644
--- a/modules/EchofonSign.jsm
+++ b/modules/EchofonSign.jsm
@@ -160,7 +160,7 @@ EchofonSign.getSignatureForSyncServer = function(str)
     var com = Cc['@naan.net/twitterfox-sign;1'].getService(Ci.nsITwitterFoxSign);
     var sig = com.sign(str);
     var toHexString = function(charCode) { return ("0" + charCode.toString(16)).slice(-2); };
-    return [toHexString(sig.charCodeAt(i)) for (i in sig)].join("");
+    return sig.map((e) => toHexString(sig.charCodeAt(e))).join("");
   }
   else {
     return signByLibrary(str);
diff --git a/modules/EchofonSync.jsm b/modules/EchofonSync.jsm
index 9787151..c962b57 100644
--- a/modules/EchofonSync.jsm
+++ b/modules/EchofonSync.jsm
@@ -364,7 +364,7 @@ EchofonSync.prototype = {
       return null;
     }
     var toHexString = function(charCode) { return ("0" + charCode.toString(16)).slice(-2); };
-    return [toHexString(sig.charCodeAt(i)) for (i in sig)].join("");
+    return sig.map((e) => toHexString(sig.charCodeAt(i))).join("");
   },
 
   onLoadSync: function(req, user, callback) {