gistfile1.txt

let str = "aAbBcCddDfgGFj";

function isUpperCase(c){
  return c === c.toUpperCase();
}

let strArr = str.split("");
console.log(strArr)

let answer = "";
let strSet = new Set();
for(let i = 0; i<strArr.length;i++){
  if(isUpperCase(strArr[i])){
    if(str.includes(strArr[i].toLowerCase())) {
      strSet.add(strArr[i].toUpperCase())
    }
  } else {
    if(str.includes(strArr[i].toUpperCase())) {
      strSet.add(strArr[i].toUpperCase())
    }
  } 
}

let strSetArr = [...strSet]
strSetArr.sort((a,b)=>a.localeCompare(b))
  alert(strSetArr[strSetArr.length-1])
console.log(answer)