Beacuse of event.keyCode is deprecated. It means we are not supposed to use that code in our project. Then how can we change code properly. Before. document.addEventListener("keyup", (e) => { if (e.keyCode == 37) { some code... } })It should be updated like this. document.addEventListener("keyup", (e) => { if (e.key == "ArrowLeft") { some code... } })Then we have one question left, how can we kn..