keyCode values for numeric keypad?

The keycodes are different. Keypad 0-9 is Keycode 96 to 105

Your if statement should be:

if ((e.keyCode >= 48 && e.keyCode <= 57) || (e.keyCode >= 96 && e.keyCode <= 105)) { 
  // 0-9 only
}

Here's a reference guide for keycodes

-- UPDATE --

This is an old answer and keyCode has been deprecated. There are now alternative methods to achieve this, such as using key:

if ((e.key >= 48 && e.key <= 57) || (e.key >= 96 && e.key <= 105)) { 
  // 0-9 only
}

Here's an output tester for event.key, thanks to @Danziger for the link.

******************* Don't use KEYCODE !!!! ******************

The problem with keyCode is to avoid the combined keys with the numbers on top of keyboard, we must add a check on the key "Shift" and "Alt" to avoid special characters such as e @ & " { } ...

A simplest solution is to convert e.key to a number and check if the conversion gives NaN!

let key = Number(e.key)
if (isNaN(key) || e.key === null || e.key === ' ') {
  console.log("is not numeric")
}
else {
  console.log("is numeric")
}

Be careful if e.key is null or a space, it gives 0 !

Number(5)         // => 5
Number('5')       // => 5
Number(null)      // => 0 
Number(' ')       // => 0
Number('chars')   // => NaN
Number(undefined) // => NaN

You can simply run

$(document).keyup(function(e) {
    console.log(e.keyCode);
});

to see the codes of pressed keys in the browser console.

Or you can find key codes here: https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/keyCode#Numpad_keys

keyCode is different for numbers on numeric keypad and numbers on top of keyboard.

keyCodes :

numbers on top of keyboard ( 0 - 9 ) : 48 - 57
numbers on numeric keypad ( 0 - 9 ) : 96 - 105

JavaScript condition :

if((e.keyCode >= 48 && e.keyCode <=57) || (e.keyCode >= 96 && e.keyCode <=105)) { 
    // entered key is a number
}

Reference for all keycodes ( with demo ) : http://www.codeforeach.com/javascript/keycode-for-each-key-and-usage-with-demo