JSFiddle - React, Tailwind, and code Playground

HTML

<div class="item">
  <div class="foo"></div>
  <div class="bar">
     <div class="slide"></div>
     <div class="slide"></div>
     <div class="slide"></div>
  </div>
</div>

<div class="item">
  <div class="foo"></div>
  <div class="bar">
     <div class="slide"></div>
     <div class="slide"></div>
  </div>
</div>

JavaScript

var counts = $(".item").map(function() {
    return $(this).find(".slide").length;
}).get();

alert(counts);