change words at particular location in javascript string

var str = "Hello world";

str.charAt(2)="P"//instead of l i am assigning the value P

console.log(str)

I want to replace some Text from the string by checking some coditions but i am getting error i also tried replace function but that return nothing does no changes in the string

>Solution :

You need to make an array out of the string to replace by index.
The following should do the trick.

const changeChar = (str, newValue, index) => {
  let splitted = str.split("");
  splitted[index] = newValue;
  return splitted.join("");
}