Find duplicate IDs

by Scott Kaye

HTML

<div id="one"></div>
<div id="two"></div>
<div id="three"></div>
<div id="two"></div>
<div id="two"></div>
<div id="three"></div>

JavaScript

console.log([...document.querySelectorAll("*")]
	.map(n => n.id)
  .filter(Boolean)
  .filter((n, _, all) => {
  	return all.filter(a => a === n).length > 1;
  })
  .filter((n, i, all) => all.indexOf(n) === i)
  .reduce((a, c) => {
  	a[c] = [...document.querySelectorAll("#" + c)];
  	return a;
  }, {}));