document.cookie="MYBIGCOOKIE=1";
Your cookies would look like:
"MYBIGCOOKIE=1; PHPSESSID=d76f00dvgrtea8f917f50db8c31cce9"
first of all read all cookies:
var read_cookies = document.cookie;
then split all cookies with ";":
var split_read_cookie = read_cookies.split(";");
then use for loop to read each value. Into loop each value split again with "=":
for (i=0;i<split_read_cookie.length;i++){
var value=split_read_cookie[i];
value=value.split("=");
if(value[0]=="MYBIGCOOKIE" && value[1]=="1"){
alert('it is 1');
}
}