JSFiddle - React, Tailwind, and code Playground

JavaScript

if(!String.prototype.myreplace){
    String.prototype.myreplace = (function(obj){
        return this.replace(/[a-z]{1,1}/gi,function(a,b){
           var r = obj[a.toLowerCase()] || a;
            return a.charCodeAt(0) > 96? r.toLowerCase() : r.toUpperCase();
        });
    });
}

var obj = {a:'q',b:'t',c:'w'};

var s = 'AbCdea';
var n = s.myreplace(obj);
console.log(n);