jQuery key code for command key

jQuery already handles that. To check if control was pressed you should use:

$(window).keydown(function (e){
    if (e.ctrlKey) alert("control");
});

The list of modifier keys:

e.ctrlKey
e.altKey
e.shiftKey

And as fudgey suggested:

e.metaKey

Might work on MAC. Some other ways here as well.

If you're not interested in the the exact moment the Command key is pressed, just whether it is pressed when another key is pressed, then you can use the metaKey property of the event.

Otherwise, for keydown and keyup events, the property you need is keyCode in all browsers. Unfortunately, the Command key does not have the same key code in all browsers, and the left and right Commands have different values in WebKit (91 and 93 respectively). I can't see an easy way of detecting these keys without some kind of browser sniff, but there may be one. The which property definitely won't help you.

For more information, http://unixpapa.com/js/key.html has comprehensive coverage of key event handling in all the major browsers.

Here you go, try this live in this runnable code snippet:

$(window).on('keypress', function(event) {
  $("body").append($("<p>").text(
       "ctrlKey " + event.ctrlKey
    + "; altKey " + event.altKey
    + "; metaKey " + event.metaKey
    + "; shiftKey " + event.shiftKey
    + "; isCommandPressed " + isCommandPressed(event)
  ));
  
});

function isCommandPressed(event) {
  return event.metaKey && ! event.ctrlKey;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<p>Focus on this select control and try pressing keys:</p>

<p><select><option>Test</option></select></p>