How it is possible to generate session id in node.js?

It is not clear what you are trying to achieve, but... a session ID is just an ID! You generate it however you want. There are no requirements except for uniqueness. It is a good idea though to make it secure. For example this function may be your session id generator:

var crypto = require('crypto');

var generate_key = function() {
    var sha = crypto.createHash('sha256');
    sha.update(Math.random().toString());
    return sha.digest('hex');
};

You call generate_key() and you check whether it exists in a database. If it does you call it again and so on and so on.