freecodecamp : Missing Letters

missing_letter.js

function fearNotLetter(str) {
  var charCode = String.charCodeAt(str[0]);
  
  for(var i = 1; i < str.length; i++){
    charCode++;    
    if(str[i] != String.fromCharCode(charCode).toLowerCase()){
      return String.fromCharCode(charCode);
    }     
  }
}

fearNotLetter("abce");