How to find the first duplicate element of an array in JavaScript?

q2b43jv.js

function firstDupe(arr) {
  return arr.findIndex((item, index) => arr.lastIndexOf(item) !== index)
}

console.log( firstDupe([8,6,7,5,3,0,9])); // --> -1 = !exist

console.log( firstDupe(["L", "E", "T", "T", "E", "R"])); // --> 1 = E

// https://stackoverflow.com/questions/39346182/javascript-how-to-find-first-duplicate-value-and-return-its-index