Guide to contributing in lib/ folder?

[Ginden]

As I'm working on pull request for #16178, I noticed that limited usage of certain ES2015+ features in lib/**/*.js code. My thoughts were confirmed while reading doc/guides/writing-tests.md.

For performance considerations, we only use a selected subset of ES.Next features in JavaScript code in the lib directory.

I think that publishing guide "what is preferred in codebase" would be preferred, because I was not able any description of such "selected subset".

Things that I would like to see:

• class vs. util.inherits
• closures
• _* modules - what is difference between them and internal/* modules?
• const vs. var vs. let
• arrow functions

[Fishrock123]

I had a start at #6885 but never got very far with it. Maybe it is of some use but quite probably outdated in some ways.

[Trott]

Many of these issues are now handled by our lint rules.

It seems like perhaps this should be closed. Feel free to re-open (or leave a comment requesting that it be re-opened) if you disagree. I'm just tidying up and not acting on a super-strong opinion or anything like that.