[bitpow]

It's because there is always lag between the latest .js spec and what is supported by browsers. So transpilers (like babel) have to dumb it down from fancyNew.js to somethingIE6MightRun.js

It is not realistic/optimal to try to write your javascript to be supported by all browsers or runtime clients. It's better to write using the latest spec, and just have it dumbed down for you by transpilers at build time

Some more explanation here: https://www.excella.com/insights/typescript-vs-es6-vs-es2015