find the first recurring character in a string

firstDupe.js

// Given a string, return the first recurring character in it, or null if there is no recurring chracter. For example, given the string "acbbac", return "b". Given the string "abcdef", return null.

const arr = "acbbac".split("");
const arr2 = "abcdef".split("");

function findFirstDuplicate(arr){
  let item = null;

  for (var i = 0; i < arr.length; i++) {
    if (arr.indexOf(arr[i]) !== i) {
      item = arr[i];
      break;
    }
  }

  return item;
}


console.log("first dupe", findFirstDuplicate(arr));
console.log("no dupe", findFirstDuplicate(arr2));