Ejercicios Realizados en CodeFights.com

firstNotRepeatingCharacter.js

function firstNotRepeatingCharacter(s) {
  /*Se define un array al cual se irá sacando información*/
  let length = s.length,result = [], seen = new Set(), notSeen = new Set();
  let notOcurrance = "_";
  for (let index = 0; index < length; index++) {
    let value = s[index];
    if (seen.has(value)){
      notSeen.delete(value);
    }else{
      notSeen.add(value);
    }
    seen.add(value);
  }
  if (notSeen.size > 0) {
    /*Se obtiene el valor del primer Set*/
    var it = notSeen.values();
    var first = it.next();    
    notOcurrance = first.value;
  }
  console.log(notOcurrance);
  return notOcurrance;
}