HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>scrollsnapchanging detect</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">checking…</pre>
  <script>
    var box = document.getElementById("snap-box");
    var out = document.getElementById("out");
    if ("onscrollsnapchanging" in window) {
      out.textContent = "supported — scroll slowly";
      box.addEventListener("scrollsnapchanging", function (event) {
        var t = event.snapTargetBlock;
        out.textContent = t
          ? "pending: " + (t.dataset.label || t.textContent.trim())
          : "pending (no block target)";
      });
    } else {
      out.textContent = "scrollsnapchanging not supported";
    }
  </script>
</body>
</html>

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Element scrollsnapchanging — feature detect (Try It)