HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>touchstart begin fallback</title>
  <style>
    #stage { height: 200px; border: 2px solid #333; touch-action: none; }
  </style>
</head>
<body>
  <div id="stage">Place two fingers here</div>
  <pre id="out">(waiting)</pre>
  <script>
    var stage = document.getElementById("stage");
    var out = document.getElementById("out");
    var startDist = 0;
    function distance(t0, t1) {
      var dx = t0.clientX - t1.clientX;
      var dy = t0.clientY - t1.clientY;
      return Math.hypot(dx, dy);
    }
    stage.addEventListener("touchstart", function (event) {
      if (event.touches.length === 2) {
        startDist = distance(event.touches[0], event.touches[1]);
        out.textContent = "pinch began (touchstart) dist=" + startDist.toFixed(1);
      }
    }, { passive: true });
    stage.addEventListener("touchmove", function (event) {
      if (event.touches.length === 2 && startDist > 0) {
        var scale = distance(event.touches[0], event.touches[1]) / startDist;
        out.textContent = "live scale≈" + scale.toFixed(3);
      }
    }, { passive: true });
    if ("GestureEvent" in window) {
      stage.addEventListener("gesturestart", function (event) {
        out.textContent = "gesturestart scale=" + event.scale.toFixed(3);
      });
    }
  </script>
</body>
</html>

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Element gesturestart — touchstart fallback (Try It)