<p>Right-click here for the document context menu</p>
<button id="elem">Right-click here for the button context menu</button>

<script>
  elem.oncontextmenu = function(event) {
    event.preventDefault();
    alert("Button context menu");
  };

  document.oncontextmenu = function(event) {
    event.preventDefault();
    alert("Document context menu");
  };
</script>