Board Thread:General Coding Help/@comment-29826282-20161230193523/@comment-11733175-20170102222929

I did some digging, and it would appear that the code used to validate and minify JS is github:Wikia/app/blob/dev/includes/libs/jsminplus.php. Looking through the code, it would appear that the parser implements an ES3 parser which lists a number of keywords, including const. Said list however, which is drawn directly from the ES3 specification (or technically ES2), does not include let, which is added as part of the ES5 specification.

Further investigation indicates JSMinPlus apparently hasn't been updated since 2011, so there's nothing to upgrade it to.