JSFiddle - React, Tailwind, and code Playground

by bradleytrager

HTML

<ul id="theList">
  <li>6</li>
  <li>3</li>
  <li>1</li>
  <li>4</li>
  <li>7</li>
  <li>5</li>
  <li>1</li>
  <li>4</li>
  <li>2</li>
  <li>8</li>
  <li>9</li>
</ul>

JavaScript

/*
Write an algorithm that will find the first duplicate in a list on the page.
For example we would return 4, for the list above.
*/
let array = document.getElementById("theList").getElementsByTagName("li");
let justNumbers = [];

for (let i = 0; i < array.length; i++) {
  justNumbers.push(array[i].innerHTML);
}

function duplicateNumber(arr) {
  return arr.find((k, i) => arr.indexOf(k) !== i);
}

console.log(duplicateNumber(justNumbers));