Well, I have found outside link this
The other way (introduced in ECMAScript 5) is to treat the string as
an array-like object, where individual characters correspond to a
return 'cat'; // returns "a"
For character access using bracket notation, attempting to delete or
assign a value to these properties will not succeed. The properties
involved are neither writable nor configurable.
so, it helped me understand this exercise, I mean exercise is simple, but to understand it properly and reuse it in a future. I do believe it could be mentioned somewhere for others ...