I am using Pega 8.5.1 and I like to customise a  javascript file by saving in to my application rule.

The javascript file after saving in to my ruleset is throwing validation errors (no changes from me yet) like "Error on line 151: Parsing error: The keyword 'let' is reserved".

From the error I suspect that the compiler used was ECMA 5 where as the file is already in ECMA 6 syntax. 

I like to understand below things:

1. How Pega was able to ship ECMA 6 syntax based file where as compiler is ECMA 5.

2. Can I manually configure to compile it in ECMA 6.

The fallback option as of now is to do online conversion from ECMA 6 to 5 using babel like tools but I dont prefer that option as the file changes completely. Thank you.