How to create a session using JavaScript?

You can store and read string information in a cookie.

If it is a session id coming from the server, the server can generate this cookie. And when another request is sent to the server the cookie will come too. Without having to do anything in the browser.

However if it is javascript that creates the session Id. You can create a cookie with javascript, with a function like:

function writeCookie(name,value,days) {
    var date, expires;
    if (days) {
        date = new Date();
        date.setTime(date.getTime()+(days*24*60*60*1000));
        expires = "; expires=" + date.toGMTString();
            }else{
        expires = "";
    }
    document.cookie = name + "=" + value + expires + "; path=/";
}

Then in each page you need this session Id you can read the cookie, with a function like:

function readCookie(name) {
    var i, c, ca, nameEQ = name + "=";
    ca = document.cookie.split(';');
    for(i=0;i < ca.length;i++) {
        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 '';
}

The read function work from any page or tab of the same domain that has written it, either if the cookie was created from the page in javascript or from the server.

To store the id:

var sId = 's234543245';
writeCookie('sessionId', sId, 3);

To read the id:

var sId = readCookie('sessionId')