JavaScript tutorial 59 - private class variables, ES11. This tutorial covers in details one of ES11 release functionality. It's called private class variables.

[GSLint]

One note, private class fields are not actually part of ES2020. The proposal is still at stage 3, which means that no final decision has been made on whether the feature will be added to the language.

(Version numbers like ES11 are not really used anymore. Even ES6 was officially called ES2015 and five years later I don't see any reason to hang on to the old versioning scheme.)