ECMAScript 2015 - 2020 are now officially supported as both input and output language for the Closure Compiler. Language support includes type checking, optimization and down-leveling (transpilation) to other language levels where it is practical to do so.

If you find that the Closure Compiler does not support your favorite feature, please file an issue. Please be aware that Closure Compiler trailing the official ECMAScript specification and does not generally support proposals prior to their acceptance into the standard.

Please see: supported features

Current draft Spec: https://tc39.github.io/ecma262/ Finished proposals: https://github.com/tc39/proposals/blob/master/finished-proposals.md