http://www.freecodecamp.com/dannycoder 's solution for Bonfire: Missing letters

bonfire-missing-letters#.js

// Bonfire: Missing letters
// Author: @dannycoder
// Challenge: http://www.freecodecamp.com/challenges/bonfire-missing-letters#
// Learn to Code at Free Code Camp (www.freecodecamp.com)

function fearNotLetter(str) {
    
  //detect if the letter is missing
  if(str.length === (str.charCodeAt(str.length - 1) - str.charCodeAt(0) + 1 )){
    
        return undefined;
    
  } else {
    
      //track down the missing letter 
      for(var i = str.charCodeAt(0), j = 0; i <= str.charCodeAt(str.length -1); i++, j++) {
           
           //return the missing letter
           if(String.fromCharCode(str.charCodeAt(j)) !== String.fromCharCode(i)) {
               return String.fromCharCode(i);
           }
      }
  }
}

fearNotLetter("abce");