Liens utiles

TEST#1

let checker = (arr, target) => target.toLowerCase().split("").every(v => arr.toLowerCase().split("").includes(v));

function findUniq(arr) {
  const length = arr.length-1;
    let count = 0;
  for(let i = length; i > -1; i--) {
    if(checker(arr[i], arr[i-1])) {
      count += 1;
      continue;
    };
    return count == 0 && !checker(arr[i], arr[i-2]) ? arr[i] : arr[i-1];
  }
}

TEST#2

let checker = (arr, target) => target.toLowerCase().split("").every(v => arr.toLowerCase().split("").includes(v));

function findUniq(arr) {
  
  if(arr.filter(w => w.toLowerCase() !== w.toUpperCase()).length == 1) {
    return arr.filter(w => w.toLowerCase() !== w.toUpperCase())[0];
  }
  
  const length = arr.length-1;
  let count = 0;
  for(let i = length; i > -1; i--) {
    if(checker(arr[i], arr[i-1])) {
      count += 1;
      continue;
    };
    return count == 0 && !checker(arr[i], arr[i-2]) ? arr[i] : arr[i-1];
  }
}