JSFiddle - React, Tailwind, and code Playground

JavaScript

var testString = 'heLLoWorld';
    var newString  = '';
    for(var i =0; i< testString.length; i++){
        if(/^[A-Z]/.test(testString[i])){
             newString+= testString[i].toLowerCase();
        } else {
             newString+= testString[i].toUpperCase();
        }
    }
    
    alert(newString);