Increment or decrement textbox value using arrow up & down keyboard keys

gistfile1.js

function change(element, increment) {
    var $el = $(element),
        elValue = parseInt($el.val(), 10),
        incAmount = increment || 1, 
        newValue = elValue + incAmount;
 
    if ((newValue) > -1) {
        $el.val(newValue);
    }
}
 
$('input.quantity').keydown(function (e) {
    var keyCode = e.keyCode || e.which,
        arrow = { left: 37, up: 38, right: 39, down: 40 };
 
    switch (keyCode) {
        case arrow.up:
            change(this, 1);
            break;
        case arrow.down:
            change(this, -1);
            break;
    }
});