JSFiddle - React, Tailwind, and code Playground

HTML

<div id="cat-0">
  <div class="item"></div>
  <div class="item"></div>
</div>
<div id="cat-1">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div>

JavaScript

const cats = document.querySelectorAll('[id^="cat-"]');

Array.prototype.slice.call(cats).forEach((cat) => {
    const items = cat.querySelectorAll('.item');
    console.log(items.length); // => 2, 3
});