ลองตัวเอง - JavaScript Reference onshow

<!DOCTYPE html>
<html>
<head>
<style>
div {
    background: yellow;
    border: 1px solid #cccccc;
    padding: 10px;
}
</style>
</head>
<body>

<p>This example uses the addEventListener() method to attach a "show" event to a menu element.</p>

<div contextmenu="mymenu">
<p>Right-click inside this box to see the context menu!

</div>

<p><strong>Note:</strong> The show event is currently only supported in Firefox.</p>

function myFunction() {
    alert("The context menu is about to be shown");
}
</script>

</body>
</html>