title: How to check if a number that starts with a letter
body: <p>I have a code below that matches a remainder of an 8 digit number to a letter when divided by 23.</p>
  
  <pre><code>function dniLetter( dni ) {
  
  var lockup = 'TRWAGMYFPDXBNJZSQVHLCKE'
  var result = ''; 
  var remainder = dni % 23;
  result = lockup.charAt(remainder)
  
  return result; }
  </code></pre>
  
  <p>How could I improve if the number starts with a negative number (like -2) or start with a letter (A1234567)?</p>