JSFiddle - React, Tailwind, and code Playground

JavaScript

var t = 'aaaBBB';
var t2 = ''
var t2 = t.split('')

function foo(){
  for(var i = 0; i < t2.length; i++){
    if(t2[i]==t2[i].toUpperCase()){
      return t2[i].toLowerCase()
     }
      if(t2[i]==t2[i].toLowerCase()){
      return t2[i].toUpperCase()
     }
  }
}
console.log(foo())