This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.

Obsolete JavaScript features and unmaintained docs

ECMAScript Next support in Mozilla
ECMAScript Next refers to new features of the ECMA-262 standard (commonly referred to as JavaScript) introduced after ECMAScript 2015. New versions of ECMAScript specifications are released yearly. This year, the ES2016 specification will be released and the ES2017 is the current ECMAScript draft specification.