The cookie is not holding and the domain example.com cannot redirect to a.example.com when I type on the address bar. Anyone good at cookies? Any help will be very much appreciated.

$(function(){
    var city = getCookie('city');
    if(city !=null && city !=''){
    window.location.href = 'http://' + city + '.example.com';
    }
$('#citygo').change(function(){
    var city = $(this).val();
    window.location.href = 'http://' + city + '.example.com';
});
});


<select id="citygo">
    <option value="0">Select City</option>
    <option value="amsterdam">Amsterdam</option>
    <option value="newyork">New York</option>
    <option value="london">London</option>
    <option value="cardiff">Cardiff</option>
</select>