[andai]

Yes! Exactly. However:

    "use strict";

    let undef;
    let str1 = "hello";
    let str2 = str1 + undef;
    console.log(str2); // -> "helloundefined"

Whereas I propose:

    "use sane";

    let undef;
    let str1 = "hello";
    let str2 = str1 + undef; // -> Uncaught TypeError: Cannot append string and undefined
    console.log(str2); // (code never reaches this point)

(I would also settle for "Uncaught ReferenceError: Attempted use of `undef` before initialization.")

[cmrdporcupine]

Yeah, JavaScript has a pile of terrible type coercions, implicit conversions and misuse/reuse of operators. It was a bad 90s/00s trend to do this kind of thing; re-use + for string append, or coerce things into strings for convenience. Python and Ruby have their own gotchas, though not as bad.

You might enjoy this classic comedy routine: https://www.destroyallsoftware.com/talks/wat