Prefer arrow callbacks in ES6 code

[edg2s]

We should consider the following rules in ES6 code:

• prefer-arrow-callback
• arrow-body-style
• implicit-arrow-linebreak

This results in arrow functions being used for callbacks (unless they reference this or arguments, in which case they would need be converted manually). When arrow functions contain a single return statement, the body-less style is prefered (e.g. ( x ) => x + 1. Also a single line is prefered for such body-less functions.

[jdforrester]

Seems sensible. Worth posting in https://www.mediawiki.org/wiki/Manual_talk:Coding_conventions/JavaScript for visibility?