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

bonfire-missing-letters.js

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

function fearNotLetter(str) {
  var start = str.charCodeAt(0);
  var end = str.charCodeAt(str.length - 1);
  var arr = [];
  for (var i = start; i <= end; i++) {
    if (str.indexOf(String.fromCharCode(i)) === -1) {
      arr.push(String.fromCharCode(i));
    }
  }
  return arr.length ? arr.join('') : undefined;
}

fearNotLetter("abce");