title: Find missing letter in list of alphabets
body: <p>I am trying to solve the following issue: </p>
  
  <p>Find the missing letter in the passed letter range and return it. If all letters are present in the range, return undefined.</p>
  
  <p>the inputs that I will get as strings are:</p>
  
  <ul>
  <li>abce (Which should return d)</li>
  <li>bcd (which should return undefined)</li>
  <li>abcdefghjklmno (which should return i)</li>
  <li>yz (which should return undefined)</li>
  </ul>
  
  <p>my code currently looks like this:</p>
  
  <pre><code>  function fearNotLetter(str) {
    //create alphabet string
    //find starting letter in alphabet str, using str
    //compare letters sequentially
    //if the sequence doesn't match at one point then return letter
    //if all letters in str appear then return undefined
  
    var alphabet = ("abcdefgheijklmnopqrstuvwxyz");
    var i = 0;
    var j = 0;
    while (i&lt;alphabet.length &amp;&amp; j&lt;str.length) {
      i++;
      if (alphabet.charCodeAt(i) === str.charCodeAt(j)) {
        i++;
        j++;      
      }
      else if (alphabet.charCodeAt(i) !== str.charCodeAt(j)) {
        i++;
        j++;
        if (alphabet.charCodeAt(i) === str.charCodeAt(j-1)) {
          return alphabet.charCodeAt(i-1);  
        }
      }
    }
  }
  
  fearNotLetter('abce');
  </code></pre>
  
  <p>Thanks for your help as always!</p>