ffd update

by Matthew Vasallo

JavaScript

const nums = [1, 3, 4, 5, 6, 3, 1, 12, 15];

const findFirstDup = nums => {
  let numbersSeen = {};

  for (let i = 0; i < nums.length; i++) {
    if (numbersSeen[nums[i]]) {
      return nums[i];
    }
    numbersSeen[nums[i]] = true;
  }
  return -1;
};

console.log(findFirstDup(nums));