HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>onscrollsnapchanging</title>
  <style>
    #snap-box {
      width: 220px; height: 160px; overflow-y: scroll;
      border: 2px solid #333; scroll-snap-type: y mandatory;
    }
    #snap-box section {
      height: 160px; scroll-snap-align: start;
      display: flex; align-items: center; justify-content: center;
      border-bottom: 1px solid #ccc; font: 16px sans-serif;
    }
  </style>
</head>
<body>
  <div id="snap-box">
    <section data-label="Card 1">Card 1</section>
    <section data-label="Card 2">Card 2</section>
    <section data-label="Card 3">Card 3</section>
  </div>
  <pre id="out">Waiting for pending snap…</pre>
  <script>
    var box = document.getElementById("snap-box");
    var out = document.getElementById("out");
    box.onscrollsnapchanging = function (event) {
      var target = event.snapTargetBlock;
      out.textContent = target
        ? "Pending: " + target.dataset.label
        : "No block target";
    };
  </script>
</body>
</html>

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Element scrollsnapchanging — onscrollsnapchanging (Try It)