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

bonfire-missing-letters.js

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

function fearNotLetter(str) {
// Create our variables.
  var firstCharacter = str.charCodeAt(0);
  var valueToReturn = '';
  var secondCharacter = '';

  // Function to find the missing letters
  var addCharacters = function(a, b) {
    while (a - 1 > b) {
      b++;
      valueToReturn += String.fromCharCode(b);
    }

    return valueToReturn;
  };

  // Check if letters are missing in between.
  for (var index = 1; index < str.length; index++) {
    secondCharacter = str.charCodeAt(index);

    // Check if the diference in code is greater than one.
    if (secondCharacter - firstCharacter > 1) {
      // Call function to missing letters
      addCharacters(secondCharacter, firstCharacter);
    }

    // Switch positions
    firstCharacter = str.charCodeAt(index);
  }

  // Check whether to return undefined or the missing letters.
  if (valueToReturn === '')
    return undefined;
  else
    return valueToReturn;
}

fearNotLetter("abce");