How come the following code doesn't change the retrieved letters into upper case?

Question

The goal of this was to change the retrieved letters into upper case. I know this works if I stored lines 6 and 7 inside a variable then replace the letters inside the variable called string, but I wanted to know why this code doesn't work instead.

Answer 1

JavaScript strings are not mutable, meaning you can't change the value of a character at a certain index. You'll need to make a new string made of these values.

let string = 'lowercasestring'; // Creates a variable and stores a string.
let letterOne = string.indexOf('l'); // Should store as 0.
let letterTwo = string.indexOf('s'); // Should store as 7.

// mutiple lines for readability
string = string[letterOne].toUpperCase() +
         string.slice(letterOne + 1, letterTwo) +
         string[letterTwo].toUpperCase() +
         string.slice(letterTwo + 1);

console.log(string);

Output:

"LowercaSestring"