FindBugInISBN.js

function isbn13(input){
  let isbn = Array.from(input);
  // console.log(isbn)
  isbn[9] = isNaN(isbn[9])? 10 : isbn[9];
  isbn = convertForChecking(isbn);
  if(isbn.reduce((a, b) => Number(a) + Number(b), 0) % (isbn.length === 10 ? 11 : 10) === 0){
    return isbn.length === 13 
      ? 'Valid' 
      : convertToIsbn13(input)
  }
    return 'Invalid';
}

function convertToIsbn13(input){
  input = '978' + input.substring(0, input.length - 1);
  let check = input.split('');
  check = convertForChecking(check);
  check = check.reduce((a,b) => Number(a)+Number(b), 0);
  input += 10 - (check % 10);
  return input;
}

function convertForChecking(isbn){
  let length = isbn.length;
  // console.log(isbn, length);
  return length === 10 
    ? isbn.map((digit, index) => !isNaN(digit) ? digit * (length - index) : 10 * (length - index))
    : isbn.map((digit, index) => digit * (index % 2 !== 0 ? 3 : 1));
}