Question

Is there a way that we can detect which side the Alt key was pressed on, i.e. distinguish between left or right Alt? I saw somewhere that it’s possible with IE with the altLeft and altRight properties of the Event object. If that is correct, how can we detect it in Firefox with JavaScript?

This is how it works in IE for altLeft:

window.onload = function(){
  document.getElementById('id').onkeydown = function(){
    alert(window.event.altLeft);
  }
}

Answer 1

2015 answer

DOM3 added a location property of keyboard events (see also MDN) (earlier versions had a keyLocation property instead) which does what you want and is implemented in recent versions of all major browsers.

Demo:

document.getElementById("ta").addEventListener("keydown", function(e) {
  var keyLocation = ["Standard", "Left", "Right", "Numpad", "Mobile", "Joystick"][e.location];
  var message = "Key '" + (e.key || e.keyIdentifier || e.keyCode) + "' is down. Location: " + keyLocation;
  this.value += "\n" + message;
  e.preventDefault();
}, false);

<textarea id="ta" rows="10" cols="50">Click on here and press some modifier keys such as Shift</textarea>

2011 answer

No. In general, it is impossible to distinguish between left and right modifier keys in a cross-browser way. The shiftLeft, shiftRight, ctrlLeft, ctrlRight, altLeft, altRight properties of window.event are all IE only and no equivalent exists in other browsers.

DOM3 added a location property of keyboard events (earlier versions had a keyLocation property instead) but Firefox does not implement this.