HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>startViewTransition async</title>
  <style>
    body { font-family: sans-serif; margin: 1rem; }
    #widget { padding: 1rem; min-height: 3rem; background: #ede9fe; border-radius: 8px; }
    pre { font-family: monospace; }
  </style>
</head>
<body>
  <div id="widget">Loading…</div>
  <button type="button" id="load">Load data</button>
  <pre id="out">—</pre>
  <script>
    var widget = document.getElementById("widget");
    document.getElementById("load").addEventListener("click", function () {
      function run() {
        if (!widget.startViewTransition) {
          widget.textContent = "Title from API (instant)";
          document.getElementById("out").textContent = "No API — instant update";
          return;
        }
        widget.startViewTransition(function () {
          return new Promise(function (resolve) {
            document.getElementById("out").textContent = "Fetching…";
            setTimeout(function () {
              widget.textContent = "Title from API (animated)";
              resolve();
            }, 600);
          });
        }).finished.then(function () {
          document.getElementById("out").textContent = "Transition after async load";
        });
      }
      run();
    });
  </script>
</body>
</html>

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Element.startViewTransition — async callback (Try It)