Google Apps Script Javascript Standard Support

According to the docs Apps Script is based on JavaScript 1.6.

This table explains the correlation to the ES standards.

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.

In the native editor it appears you are limited to an earlier version of JS (~ES5).

There is an official tool called "Clasp" though, which lets you write typescript (.ts) files with ES6+ syntax and then compile & upload your files to Drive with clasp push --watch.

Alternatively, you can use Webpack in your local setup as detailed in this answer.