[SuperZen]

потом обнаружил, что 43й ФФ не поддерживает "let"

...
Run your index.html in various browser such as Chrome,Firefox and Edge Browser, if your getting error message means the browser is not accepting ES6 javascript
So we have to convert our ES6 into ES5 script format.
How ??
We have use babel for converting ES6 to ES5....
взял тут https://medium.com/@zural143/basic-w...l-dc66e72c86c6