Check RNA Nucleotides

[RazvanBugoi]

https://www.codewars.com/kata/5a3325b4b6486ab367000064/train/javascript

[RazvanBugoi]

function checkNucleotides(sequence) {
  let output = '';
  let firstLetter = '';
    for (let i=0; i< sequence.length; i++) {
      if(sequence[i] == 'A' || sequence[i] == 'G' || sequence[i] == 'C' || sequence[i] == 'U') {
     output += sequence[i];
} else {
        firstLetter += sequence[i];
}
}
  if (output.length == sequence.length)  return `Valid Sequence: ${output}`;
  if (output.length != sequence.length)  return `There is an invalid letter: ${firstLetter[0]}. At position: ${sequence.indexOf(firstLetter[0]) + 1}`
}