HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>mouseup button</title>
  <style>
    #stage { height: 100px; border: 2px solid #333; display: grid; place-items: center; user-select: none; }
  </style>
</head>
<body>
  <div id="stage">Try left / middle / right release</div>
  <pre id="out">(waiting)</pre>
  <script>
    var stage = document.getElementById("stage");
    var out = document.getElementById("out");
    var names = { 0: "primary", 1: "middle", 2: "secondary" };
    stage.addEventListener("mouseup", function (event) {
      event.preventDefault();
      out.textContent =
        "button=" + event.button +
        " (" + (names[event.button] || "other") + ")" +
        " buttons=" + event.buttons;
    });
  </script>
</body>
</html>

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Element mouseup — which button (Try It)