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Html and javascript cookie dying after browser window closes

I've added a darkmode setting to my website, and I made a cookie using javascript so it's saved for when you come back, everything works like a charm until you close the browser window.
var cookie = document.cookie = 'darkmode=false; SameSite=none; Secure';
Is there a way to make the cookie stay for as long as possible?

You should add expires and/or max-age for the cookie.
Check the documentation for more information.
var cookie = document.cookie = 'darkmode=false; max-age=31536000; expires=Fri, 31 Dec 9999 23:59:59 GMT; SameSite=none; Secure';

Create cookie on closing the browser window or tab

I have a problem. I want to create a cookie in JavaScript and I do it like the following:
document.cookie = "type=banner;expires=Thu, 25 Nov 2020 08:50:17 GMT";
Now the problem is that when I close the browser I want this cookie to expire and also to create a new one just like this document.cookie = "type=popup;expires=Thu, 25 Nov 2020 08:50:17 GMT";
How can I achieve the following functionality in JavaScript?

First of all you can listen for the beforeunload event. This happens "when the window, the document and its resources are about to be unloaded." BeforeUnloadEvent - Web APIs. A cookie can be deleted by setting its expires to zero Document.cookie - Web APIs. And then you can add the new one.
window.addEventListener('beforeunload', e => {
//"delete" the old cookie
document.cookie = "type=banner;expires=0";
//add the new cookie
var expires = new Date();
expires.setHours(expires.getHours()+2); // expires in two hours
document.cookie = `type=popup;expires=${expires.toUTCString()}`;
});

Is it safe to set cookie with unquoted path?

I would like to execute the following JavaScript to set a browser cookie:
document.cookie = "name=value;path='/'"
This works fine in Firefox, Chrome, and Safari. It does not work in IE, however. Removing the path part or unquoting '/' seems to set the cookie correctly in IE. I'm not an expert on the cookie spec. All of the guides online seem to quote the path. Is it required or optional to quote the path?

According to W3Schools correct syntax is following:
With a path parameter, you can tell the browser what path the cookie
belongs to. By default, the cookie belongs to the current page.
document.cookie = "username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC; path=/";
http://www.w3schools.com/js/js_cookies.asp

Yes it is safe. it is the right way to do it when using plain old javascript.
document.cookie = "username=John Doe; expires=Thu, 18 Dec 2013 12:00:00 UTC; path=/";
or via Jquery plugin jquery.cookie
$.cookie('name', 'value', { expires: 7, path: '/' });
//or
$.cookie('name', 'value', { path: '/' });

Javascript delete cookie before reload or redirect

I need to delete a cookie and then do a redirect. However the cookie doesn't get deleted until the redirect is processed. The problem is that if the cookie still exists at the time the redirect is executed, the redirect is intercepted and sent to a page other than the one intended. (weird, I know; long story)
Is it possible to trick the browser and force the deletion of the cookie before the redirect?
jQuery('div#panelD').click(function(){
document.cookie = 'sharedsession=; expires=Thu, 01 Jan 1970 00:00:01 GMT; Domain=.example.com; path=/';
window.location.href = "www.example.com/x";
});

I discovered if I perform an AJAX call for anything, it qualifies as a refresh for the purposed of deleting cookies.
var fakeAjax = new XMLHttpRequest();
var anything = fakeAjax.responseText;
fakeAjax.open("GET","ajax_info.txt",false); // file doesn't actually exist
fakeAjax.send();
Note the "false" in the Open line. Asynchronous has to be set to false (or some other delay) to allow time for the new info to come back and the cookie to be deleted.
UPDATE: IE doesn't like requesting responseText from a file that doesn't exist so just remove that line completely. Other browsers seem to be fine with or without it.

Understanding this go back logic

I saw this go back code in a website and I am trying to use it on my website. I am having trouble in understanding how the toURl and refUrl are being generated in it. Please guide about their structure. Thanks
var backtriggered = false;
setTimeout('goBack()',timer);
function goBack() {
if(backtriggered) return false;
backtriggered = true;
toURL=getWelcomeCookie('toURL');
refURL = getWelcomeCookie('refURL');
var reg = /(.*)\/sites\/(.*)\/(\d{4})\/(\d{2})\/(\d{2})\/(.*)\/?/;
if(reg.test(toURL) == false){
document.cookie="toURL"+ "=" +escape(toURL)+";path=/; domain=.mycsnippets.com; expires=Thu, 01-Jan-1900 00:00:01 GMT";
document.cookie="refURL"+ "=" +escape(toURL)+";path=/; domain=.mycsnippets.com; expires=Thu, 01-Jan-1900 00:00:01 GMT";
}
location.href=toURL;
}

document.cookie="toURL"+ "=" +escape(toURL)+";path=/; domain=.mycsnippets.com; expires=Thu, 01-Jan-1900 00:00:01 GMT";
This line puts a value in the brower's cookie store. For more information I think you should read this first: http://en.wikipedia.org/wiki/HTTP_cookie

There are two ways to set cookies: through server response, and through javascript.
What this code does is set cookies using javascript which say where the visitor has been and where he is going before setting the url to the target url (toURL). This way next page when it is called it knows to go back to the url which was set in the cookies as the previous url.