Javascript Cookie with no expiration date

You can do as the example on Mozilla docs:

 document.cookie = "someCookieName=true; expires=Fri, 31 Dec 9999 23:59:59 GMT";

P.S

Of course, there will be an issue if humanity still uses your code on the first minute of year 10000 :)

There is no syntax for what you want. Not setting expires causes the cookie to expire at the end of the session. The only option is to pick some arbitrarily large value. Be aware that some browsers have problems with dates past 2038 (when unix epoch time exceeds a 32-bit int).

Nope. That can't be done. The best 'way' of doing that is just making the expiration date be like 2100.