Mutations challenge. My code is returning the wrong boolean on 3rd, 5th and last argument

My code does not return the right boolean on the 3rd, 5th and last arguments. I’m not sure why can anyone give me some insight?

function mutation(arr) {
  var tester = arr[1].toLowerCase();
  var against = arr[0].toLowerCase();

  var testerArr = tester.split('');
  var againstArr = against.split('');

  for (var i = 0; i < tester.length; i++) {
    var io = testerArr.indexOf(againstArr[i]);
  }

  if (io === -1) {
    return false;
  } else {
    return true;
  }
}

Mutations Challenge