javascript - How come the key value of a js key minus a number becomes NaN

Question

This is the code and browser screenshot. The first one is number, the second one is also number, but the third one is NaN.
I want to get a key combination, such as Alt 1...Alt 9 and then decrease the key value. Go to a number that corresponds to the value of the option of select
but it keeps reporting NaN

$(function(){
    $(document).keydown(function(event){
        if(event.keyCode <=57 && event.keyCode >= 49 && event.altKey){
            var selectval = (event.keyCode - 48);
            console.log(typeof(event.keyCode));
            console.log(typeof(selectval));
            console.log(selectval);
        }
    });
})

Answer 1

Answer 2

var selectval = (event.KeyCode - 48); => var selectval = (event.keyCode - 48);

是KeyCode还是keyCode？

Answer 3

There’s nothing wrong with writing it in native language

document.addEventListener('keydown',function(event){
    if(event.keyCode <=57 && event.keyCode >= 49 && event.altKey){
            var selectval = (event.keyCode - 48);
            console.log(typeof(event.keyCode));
            console.log(typeof(selectval));
            console.log(selectval);
        }
},false);

Answer 4

Use event.ctrlKey, event.shiftKey, event.altKey to determine whether the ctrl key, shift key and alt key are pressed

Answer 5

After changing several environments, this situation finally occurred. They are all the highest version of Google. I don’t know how there are several computers without undefined

The keyCode received by the new variable is undefined;
As for why I don’t understand it yet, can someone please explain the mechanism

Answer 6

jQuery will help you with compatibility processing, please use event.which代替event.keyCode.

No need for jQuery:

// 获取用户单击键盘的“键值”
var keyCode = event.keyCode ? event.keyCode : event.which ? event.which : event.charCode;