How can use “getModifierState” on focus event with JS?

问题

I'm trying to get caps lock status on focus event of input this is my code

$('#Input')[0].addEventListener('focus', function (key) {
                         if (key.originalEvent.getModifierState("CapsLock"))
                            //do something...
                        else {
                            //do something..
                        }
                });

But i'm getting this error "Cannot read property 'getModifierState' of undefined", and I don't know what I'm doing wrong. Some advice or link for to read thanks in advance.

回答1:

There are different types of event object. getModifierState is only defined for KeyboardEvent (such as a keydown) and MouseEvent (such as a click); focus is neither.

You might look at using click and remembering whether you already had focus (e.g., so you don't repeat the action if the click is in an already-focussed element). Ugh, no, that's a terrible suggestion. There are lots of ways your element might get focus other than clicks.

来源：https://stackoverflow.com/questions/40156072/how-can-use-getmodifierstate-on-focus-event-with-js