// If you want to get ALL occurrences (g), be case insensitive (i), and use boundaries so that it isn't a word within another word (\\b):

re = new RegExp(`\\b${replaceThis}\\b`, 'gi');

// example:

let inputString = "I'm John, or johnny, but I prefer john.";
let replaceThis = "John";
let re = new RegExp(`\\b${replaceThis}\\b`, 'gi');
console.log(inputString.replace(re, "Jack")); // I'm Jack, or johnny, but I prefer Jack.