Allow only numbers and ctrl+a , ctrl+v , ctrl+c to a textbox

Try with event.keyCode and event.metaKey like this.

$(document).ready(function() {
  $(".allow_only_numbers").keydown(function(e) {
    // Allow: backspace, delete, tab, escape, enter and .
    if ($.inArray(e.keyCode, [46, 8, 9, 27, 13, 110, 190]) !== -1 ||
      // Allow: Ctrl+A,Ctrl+C,Ctrl+V, Command+A
      ((e.keyCode == 65 || e.keyCode == 86 || e.keyCode == 67) && (e.ctrlKey === true || e.metaKey === true)) ||
      // Allow: home, end, left, right, down, up
      (e.keyCode >= 35 && e.keyCode <= 40)) {
      // let it happen, don't do anything
      return;
    }
    // Ensure that it is a number and stop the keypress
    if ((e.shiftKey || (e.keyCode < 48 || e.keyCode > 57)) && (e.keyCode < 96 || e.keyCode > 105)) {
      e.preventDefault();
    }
  });
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<input type="number" class="allow_only_numbers" />

EDIT:

Remove the following code snippets from your code.

var ctrlDown = false;
var ctrlKey = 17, vKey = 86, cKey = 67;
if (e.keyCode === ctrlKey) {
    ctrlDown = true;
}

Because ctrlDown will be false while pressing C and V for copy and paste. And hence your ctr+c and ctrl+v is not working.

$(".allow_only_numbers").on("input",function (e) {
  e.target.value = e.target.value.replace(/[^0-9]/g,'')
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<input type="text" class="allow_only_numbers" />

$(".allow_only_numbers").keydown(function (e) {
        var isModifierkeyPressed = (e.metaKey || e.ctrlKey || e.shiftKey);
        var isCursorMoveOrDeleteAction = ([46,8,37,38,39,40].indexOf(e.keyCode) != -1);
        var isNumKeyPressed = (e.keyCode >= 48 && e.keyCode <= 58) || (e.keyCode >=96 && e.keyCode <= 105);
        var vKey = 86, cKey = 67,aKey = 65;
        switch(true){
            case isCursorMoveOrDeleteAction:
            case isModifierkeyPressed == false && isNumKeyPressed:
            case (e.metaKey || e.ctrlKey) && ([vKey,cKey,aKey].indexOf(e.keyCode) != -1):
                break;
            default:
                e.preventDefault();
        }
});

here is working example

https://jsfiddle.net/stwzhceL/8/

this allow you ctrl+a,ctrl+c and ctrl+v as well as arrow keys,delete key and backspace key.

plus this code covers mac as well.(which is cmd+a/cmd+c/cmd+v)