How to change client's DOM, after authenticate the client's info via the Http "Get" request?

There is a signIn/signUp system in my webApp and I use the socket.emit event for the authentication flow.

The logic is:

client clicks a signIn button, then his/her userInfo will be emit to the server-->the server receive the request, and send back the instruction after authentication via another emit---->upon receiving the instruction, the client change the DOM,disappearing login form, displaying content etc..

However, if client's userInfo is not sent through emit event but via HTTP get request. I don't know how to change the specific clients DOM to finish the logic flow.

app.get('/callback', function (req, res, next) {
   var code = req.query.code;
      client.getAccessToken(code, function (err, result) {
        var openid = result.data.openid;
          client.getUser(openid, function (err, result) {
             var userInfo = result; //this is the userInfo
            db.user.insert({username:userInfo.name,password:userInfo.password}) //I register the client here, but it is not reflected in the client's page
            res.redirect(/)  // redirect the client to the homepage,but I actually want to change the client's DOM
          // can I send something from here like socket.emit("successful",{status:"succcessful"}) to the specific client?

    });
});

});

via Wei Zheng