如果用户isLoggedin使用node和express，则限制登录页面访问

login api:

    api.post('/login', function(req, res) {

    User.findOne({ 
        username: req.body.username
    }).select('password').exec(function(err, user) {

        if(err) throw err;

        if(!user) {
            res.send({ message: "User doesn't exist."});
        } else if(user) {

            var validPassword = user.comparePassword(req.body.password);

            if(!validPassword) {
                res.send({ message: "Invalid password."});
            } else {

                /////// token

                var token = createToken(user);
                res.json({
                    success: true,
                    message: "Successfully logged in.",
                    token: token
                });
            }
        }
    });
});
    api.post('/login', function(re