https://mail.mozilla.org/pipermail/es-discuss/2014-March/036512.html

> a bit of additional clarification:  It depends upon whether the file is a module or a script file.
> If it is a module, then let-like declarations have file scope.  If it is a normal script, then let-like
> declarations are global scope (visible to multiple script files) but does not create a property on the global object.