Index: gc_core/js/helpers.js
==================================================================
--- gc_core/js/helpers.js
+++ gc_core/js/helpers.js
@@ -1,28 +1,23 @@
 
 // HELPERS
 
 "use strict";
 
-// In Firefox, there is no console.log in PromiseWorker, but there is worker.log.
-// In Thunderbird, you can’t access to console directly. So it’s required to pass a log function.
-var funcOutput = null;
 
 var helpers = {
+    // In Firefox, there is no console.log in PromiseWorker, but there is worker.log.
+    // In Thunderbird, you can’t access to console directly. So it’s required to pass a log function.
+    funcOutput: null,
+
     setLogOutput: function (func) {
-        try {
-            funcOutput = func;
-        }
-        catch (e) {
-            func(e);
-            console.error(e);
-        }
+        this.funcOutput = func;
     },
 
     echo: function (obj) {
-        if (funcOutput !== null) {
-            funcOutput(obj);
+        if (this.funcOutput !== null) {
+            this.funcOutput(obj);
         } else {
             console.log(obj);
         }
         return true;
     },
@@ -30,12 +25,12 @@
     logerror: function (e, bStack=false) {
         let sMsg = "\n" + e.fileName + "\n" + e.name + "\nline: " + e.lineNumber + "\n" + e.message;
         if (bStack) {
             sMsg += "\n--- Stack ---\n" + e.stack;
         }
-        if (funcOutput !== null) {
-            funcOutput(sMsg);
+        if (this.funcOutput !== null) {
+            this.funcOutput(sMsg);
         } else {
             console.error(sMsg);
         }
     },
 
@@ -91,15 +86,15 @@
         return obj;
     }
 }
 
 
-
 if (typeof(exports) !== 'undefined') {
+    export.funcOutput = helpers.funcOutput;
     exports.setLogOutput = helpers.setLogOutput;
     exports.echo = helpers.echo;
     exports.logerror = helpers.logerror;
     exports.inspect = helpers.inspect;
     exports.loadFile = helpers.loadFile;
     exports.objectToMap = helpers.objectToMap;
     exports.mapToObject = helpers.mapToObject;
 }