http://www.freecodecamp.com/narshe1412 's solution for Bonfire: Missing Letters

Bonfire Missing Letters.js

function fearNotLetter(str) {
  for (var i = 0, char = str.charCodeAt(0); i < str.length; i++, char++) {
    if (str.charCodeAt(i) !== char) {
      return String.fromCharCode(char);
    }
  }
}

fearNotLetter("abce");