what's wrong?

Question

why does 16 come out instead of undefined?

let arr = [2,8,16];
for(let key of arr){
console.log(arr[key]);
}

Answer 1

XPEHO3ABP

because 'of' gives value and 'in' gives key
so arr[key] = arr[2] = 16 and rest are undefined.

Do this:

let arr = [2,8,16];

for(let key in arr){
//console.log(key);
console.log(arr[key]);
}

for(let val of arr){
//console.log(key);
console.log(val);
}

Answer 2

Потому что итерация проходится по массиву