The KeyboardEvent.altKey read-only property is a Boolean indicates if the alt key (Option or ⌥ on OS X) was pressed (true) or not (false) when the event occured.
var altKeyPressed = instanceOfKeyboardEvent.altKey
"Key Pressed: " + String.fromCharCode(e.charCode) + "\n"
+ "charCode: " + e.charCode + "\n"
+ "ALT key pressed: " + e.altKey + "\n"
Press any character key,
with or without holding down the ALT key.<br />
You can also use the SHIFT key together with the ALT key.