Questioning the function of "let" when declaring a variable

[GoToLoop]

By default, JS code run by a browser uses the “sloppy mode”, which doesn’t require a variable to be declared before using it:

Let or nothing?

That behavior is nicknamed “sloppy mode”: In order to activate “strict mode”, place "use strict"; as the 1st non-comment statement in a JS file. We can also activate “strict mode” by loading a JS file as an “ECMA script module (ESM)”.

BtW, JS got various keyword commands which can be used to declare a variable:
var, let, const, function, class, import.