Google Apps Script Javascript Standard Support

Question

Simple question: Can anyone point to a resource or knows anything about which standard of Javascript is supported in Google Apps Script?

I assume naturally that ES5

Answer 1

From https://developers.google.com/apps-script/guides/services/#basic_javascript_features (January 27, 2019)

Basic JavaScript features

Apps Script is based on JavaScript 1.6, plus a few features from 1.7 and 1.8. Many basic JavaScript features are thus available in addition to the built-in and advanced Google services: you can use common objects like Array, Date, RegExp, and so forth, as well as the Math and Object global objects. However, because Apps Script code runs on Google's servers (not client-side, except for HTML-service pages), browser-based features like DOM manipulation or the Window API are not available.