That's why we introduced the strict mode ("use strict" pragma)[1]. Among other things, it prevents from accidentally declaring a global variable this way, throwing a ReferenceError.

Actually, at this point, with the prolifiration of strict mode and linters, I'd say that these old gotchas mostly belong to quizzes and spec discussions, since personally I have not seen new code written this way even in vanilla JS for years by now.

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...