event.which

event.which and event.keyCode are deprecated in modern browsers. Use .key or .code instead.

event.code

The physical key on the keyboard. Doesn't care if you are holding a modifier like Shift.

event.key

The value of the key pressed. Accounts for modifiers keys that return CAPS and alternate chars.

event.location

Some keys exist more than once on your keyboard. This provides the location of the key pressed. Try it with both shifts.