How to get all multiple keys pressed in javascript?

Question

i found this jsfiddle, that should show all keys pressed on the keyboard. The code looks fine, but it does not seem to work, at least on my keyboard: https://jsfiddle.net/HwR9N/3/

When i press all the arrow keys, only two of the keys get listed.

I work with a similar code, to create a javascript game, but i got the same problem there:

//List of all keys pressed or up
    var keymap = [];
    window.addEventListener("keydown",
        function(e){
            keymap[e.keyCode] = true;
            checkCombinations(e);
        },
    false);
    
    window.addEventListener('keyup',
        function(e){
            keymap[e.keyCode] = false;
        },
    false);

A strange thing is, that the WASD keys work all at the same time. Does anybody know a different method, that works on every keyboard? I haven't tried another keyboard yet. Im working on a HP Laptop. OS is Windows 10. Problem occurs in Chrome and Firefox.

Im thankfull for any advice.

Greets, Florian