JSFiddle - React, Tailwind, and code Playground

HTML

Look in the console.

JavaScript

Object.defineProperty(String.prototype, "map", {
    value: function(fn, thisArg = undefined) {
        let result = "";
        for (const ch of this) {
            result += fn.call(thisArg, ch);
        }
        return result;
    }
});
console.log("testing 😀".map(ch => ch.toUpperCase()));