How to first check cookie value then go to page?

<a id="linkCart" href="cart.aspx">cart</a>

var userCookie = getCookie('loggedInUser');         

function getCookie(name) {            

 var nameEQ = name + "=";             

var ca = document.cookie.split(';');             

for (var i = 0; i < ca.length; i++) {                 

var c = ca[i];                 

while (c.charAt(0) == ' ') c = c.substring(1, c.length);                 

if (c.indexOf(nameEQ) == 0) 

return c.substring(nameEQ.length, c.length);             

}             

return null;         

}         

if (userCookie == null) 

{             

window.location.href = "loginpage.aspx";        

}