javaScript获取指定的cookie值

function getCookie(cookieName) {
    var strCookie = document.cookie;
    var arrCookie = strCookie.split("; ");
    for(var i = 0; i < arrCookie.length; i++){
        var arr = arrCookie[i].split("=");
        if(cookieName == arr[0]){
            return arr[1];
        }
    }
    return "";
}

var user_id = getCookie("selected_cookie_name");