e.which

e.which is not an event, which is a property of the event object, which most people label as e in their event handlers. It contains the key code of the key which was pressed to trigger the event (eg: keydown, keyup).

document.onkeypress = function(myEvent) { // doesn't have to be "e"
    console.log(myEvent.which);
};

With that code, the console will print out the code of any key you press on the keyboard.