Ejercicios Realizados en CodeFights.com

firstDuplicate.js

function firstDuplicate(a) {
  let length = a.length,result = [], seen = new Set();
  let firstOccurance = -1;
  for (let index = 0; index < length; index++) {
    let value = a[index];
    if (seen.has(value)){
      /*Guardo primera ocurrencia y quiebro*/
      firstOccurance=value;
      break;
    }    
    seen.add(value);    
  }
  console.log(firstOccurance);
  return firstOccurance;
}