JSFiddle - React, Tailwind, and code Playground

JavaScript

var str = "This is an example sentence, thanks.";

var newString = "";
for(var i=0; i<str.length; i++)
{
   if ((i % 5) == 4) {
     newString += "X";
   } else {
     newString += str.charAt(i);
   }
}

console.log(newString);