By default, Closure compiler supports JavaScript as specified inEcma-262, edition 10 (ECMAScript 2019).

Support for ECMAScript 2020 features is in progress.

Here are some pages that we find useful for understanding the language:

• Mozilla's Core JavaScript Reference
• Differences between IE's JScript implementation and JavaScript (PDF)

Using the command-line options--language_in, the compiler can be switched to accept any version of ECMAScript down toEcma-262, edition 3 (ECMAScript 3) (PDF). See theflags page for a full list of supported--language_in values.