HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>scrollBy() — Promise</title>
</head>
<body>
  <h1>Await scrollBy()</h1>
  <p>
    <button type="button" id="step">Smooth +200</button>
    <button type="button" id="detect">Detect Promise</button>
  </p>
  <pre id="out">Click a button</pre>
  <p>Top</p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p><br><br><br><br><br><br><br><br><br><br></p>
  <p>Bottom</p>
  <script>
    (function () {
      var out = document.getElementById("out");

      document.getElementById("detect").onclick = function () {
        var test = window.scrollBy(0, 0);
        var ok = !!(test && typeof test.then === "function");
        out.textContent =
          "scrollBy returns Promise? " + ok +
          (ok ? "\nYou can await result.interrupted" : "\nScroll still works without await");
      };

      document.getElementById("step").onclick = async function () {
        var result = window.scrollBy({ top: 200, behavior: "smooth" });
        if (result && typeof result.then === "function") {
          var data = await result;
          out.textContent =
            "Smooth scroll finished\ninterrupted? " + data.interrupted +
            "\nscrollY: " + Math.round(window.scrollY || 0);
        } else {
          out.textContent =
            "Smooth scroll started (no Promise in this browser)\nscrollY: " +
            Math.round(window.scrollY || 0);
        }
      };
    })();
  </script>
</body>
</html>

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