JavaScript - From letters to digits

Question

Output is "012345". How is it letters are "transformed" in digits?

let result = "";
for(let i in "abcdef"){
result += i;
}
document.write(result);

Answer 1

its not converting, but it take the index from the array

Answer 2

no, it take the properties.
you can use for of to take the value

Answer 3

Shouldn't it take values?

Answer 4

for...of vs for...in Loops in JavaScript
https://alligator.io/js/for-of-for-in-loops/