HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>button — left only</title>
</head>
<body>
  <div id="target" style="padding:1rem;border:1px solid #888;cursor:pointer">
    Press mouse buttons here
  </div>
  <pre id="out"></pre>
  <script>
    document.getElementById("target").addEventListener("mousedown", function (e) {
      if (e.button !== 0) {
        document.getElementById("out").textContent =
          "Ignored non-primary button: " + e.button;
        return;
      }
      document.getElementById("out").textContent = "Primary (left) press";
    });
    document.getElementById("target").addEventListener("contextmenu", function (e) {
      e.preventDefault();
    });
  </script>
</body>
</html>

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MouseEvent.button — left only (Try It)