Use following :

$(document).ready(function() {
    $("#txtboxToFilter").keydown(function(event) {
        // Allow: backspace, delete, tab and escape
        if ( event.keyCode == 46 || event.keyCode == 8 || event.keyCode == 9 || event.keyCode == 27 ||
             // Allow: Ctrl+A
            (event.keyCode == 65 && event.ctrlKey === true) ||
             // Allow: home, end, left, right
            (event.keyCode >= 35 && event.keyCode <= 39)) {
                 // let it happen, don't do anything
                 return;
        }
        else {
            // Ensure that it is a number and stop the keypress
            if (event.shiftKey || (event.keyCode < 48 || event.keyCode > 57) && (event.keyCode < 96 || event.keyCode > 105 )) {
                event.preventDefault();
            }  
        }
    });
});

OR you can use

if($('#Field').val() != "") {
    var value = $('#Field').val().replace(/^\s\s*/, '').replace(/\s\s*$/, '');
    var intRegex = /^\d+$/;
    if(!intRegex.test(value)) {
        errors += "Field must be numeric.<br/>";
        success = false;
    }
} else {
    errors += "Field is blank.</br />";
    success = false;
}