25.Bonfire: Missing letters

Missing letters

function fearNotLetter(str) {
  var missing = '';
  var isMissing = false;
  for (var i = 0; i < str.length-1; i++) {
    if (str.charCodeAt(i+1) - str.charCodeAt(i) !== 1 && !isMissing) {
      missing = String.fromCharCode(str.charCodeAt(i)+1);
      isMissing = true;
    } else if (!isMissing) {
        missing = undefined;
    }
    console.log(missing);
  }
  return missing;
}

fearNotLetter('bcd');