kata-7-solution-first-duplicate.js

// ************************************************************************************
// https://www.codewars.com/kata/54f9f4d7c41722304e000bbb/javascript

// Find the first character that repeats in a String and return that character.
// firstDup('tweet') => 't'
// firstDup('like') => undefined

// This is not the same as finding the character that repeats first. In that case,
// an input of 'tweet' would yield 'e'.
// ************************************************************************************

function firstDup(str) {
  return str
    .split('')
    .filter((elem, i, arr) => arr.indexOf(elem) !== arr.lastIndexOf(elem))[0];
}