Are all variables still global unless defined not to be?
https://en.wikipedia.org/wiki/ECMAScript_version_history#ES2...
AFAIK core javascript works exactly like 25 years ago, and they haven't changed anything on it, not removing a single keyword or altering a semantics of any less used function, so your 25 years old javascript programs can still run in modern browsers.
# a.mjs
(()=>{
a = 1
})();
# in a shell
$ node a.mjs
ReferenceError: a is not defined<script scr=...>
And in the .js file: var x = 5. That x is global.