Google Apps ScriptのJavaScriptのバージョン

2020-02-05までは、baseにしているJavaScriptのバージョンは 1.6 でしたが
アップデートがあって、アロー関数なども使えるようになった。

Release Notes
https://developers.google.com/apps-script/releases#february_5_2020


Built-in Google Services からの 引用

Modern JavaScript features
Apps Script supports two JavaScript runtimes: the modern V8 runtime and an older one powered by Mozilla's Rhino JavaScript interpreter.

The V8 runtime supports modern ECMAScript syntax and features. The Rhino runtime is based on the older JavaScript 1.6 standard, plus a few features from 1.7 and 1.8. You can freely choose which runtime to use with your script, but the V8 runtime is strongly recommended.

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参考
Built-in Google Services
https://developers.google.com/apps-script/guides/services/#basic_javascript_features

バージョンとブラウザの対応表
https://ja.wikipedia.org/wiki/JavaScript#%E3%83%90%E3%83%BC%E3%82%B8%E3%83%A7%E3%83%B3%E3%81%A8%E3%83%96%E3%83%A9%E3%82%A6%E3%82%B6%E3%81%AE%E5%AF%BE%E5%BF%9C%E8%A1%A8