Javascript String firstRepeatLetter()

 // Write a JavaScript function to find the first not repeated character
//  Sample arguments : 'abacddbec' 
// Expected output : 'e' 
String.prototype.firstRepeatLetter = function (){
 var letters = this.split("");
 var letterSet = new Set();
 letters.some( function(element, index) {
  // statements//from w  ww  . j ava  2 s .  com
  if ( !letterSet.has(element) ){
   letterSet.add(element);
  }
  else{
   console.log(element);
   return false;
  }
 });
}
"fuck you".firstRepeatLetter();