Is it possible to set up an event listener in the browser to capture values returned from res.render?

Desired Behaviour

I have a Node.js/Express application that uses Azure AD B2C to login.

I want to return values returned from the server-side, OAuth style, login to the browser and save them in browser cookies (using js-cookie).

I want to do this so that I can use these values in various UI functionality.

However, the only thing that currently goes back to the browser is via res.render() (using a simple template engine):

res.render('index', { page_html: my_page_html, some_other_value: 'hello'  });

Essentially, res.render returns an html file to the browser which contains placeholder values that are replaced by the variables passed into res.render.

Is it possible to set up an event listener in the browser to capture values returned from res.render?

Or, if that is not possible, is there any other way to 'push' a value to the frontend (in parallel with res.render) so that it could be handled by frontend code?

Context

Previously, in other projects, I have made Ajax requests from the frontend to the backend and therefore had easy access to what was returned from the server, which I could then use in response handler logic and save in browser cookies etc.

However, in this instance, I am doing all my login logic from the backend code and therefore there is no specific request from frontend to backend, and no response handler in the frontend.

So I am not sure how I can pass values received in the backend (after login) to the frontend.

The authentication process follows a common OAuth work flow:

• User hits a protected endpoint - in this case the 'root' endpoint (i.e: /)
• User is forwarded to identity provider to login (in this case Azure AD B2C)
• On successful login, user is forwarded to a redirect URI, and authorisation code is received in server-side code
• Server-side code uses authorisation code to request an access token
• Access token is saved in req.session.accessToken
• On subsequent requests to protected endpoints, if req.session.accessToken is present, the request can proceed

This project does not use any JavaScript framework (eg React, Angular).

You can simply set a cookie on the response by using the callback of the res.render() method and call res.cookie() inside of it before calling res.send().

The browser will save the cookie as expected, which you can then access using js-cookie.

For reference, you need to do something like this

res.render('index', { /* locals */ }, function (err, html) {
  res.cookie('key', 'value');
  res.send(html);
);