req.session.user - Property 'user' does not exist on type 'Session' error when trying to pass session data using express-session

I am trying to make a simple login form for users to log into a website, where I then save their session data into a session cookie. I am using express-session and in their documentation it gives this as an example of setting it up:

app.post('/login', express.urlencoded({ extended: false }), function (req, res) {
  // login logic to validate req.body.user and req.body.pass
  // would be implemented here. for this example any combo works

    // store user information in session, typically a user id
    req.session.user = req.body.user

    // save the session before redirection to ensure page
    // load does not happen before session is saved
    req.session.save(function (err) {
      if (err) return next(err)
      res.redirect('/')
    })
  })
})

but in my code I keep getting an error at the 'req.session.user' part which says: "Property 'user' does not exist on type 'Session & Partial<SessionData>'" even if I am using the exact same code from the express-session documentation.

I follow all the instructions in the documentation and have this at the top of the program as well:

import session from 'express-session';

app.set('trust proxy', 1);

app.use(session({  
  name: `First_test`,
  secret: 'secret_text', 
  saveUninitialized: true, 
  resave: true,
  cookie: { 
    secure: false, 
    maxAge: 960000 
  } 
}));

When debugging I can see that inside the req.session there is indeed no 'user' property.

I have searched google and stackoverflow but haven't been able to find a solution. Most likely I have missed some small step and I am hoping someone here can help educate me.

Thanks for the help in advance.

From the @types/express-session package, we can see

    /**
     * This interface allows you to declare additional properties on your session object using [declaration merging](https://www.typescriptlang.org/docs/handbook/declaration-merging.html).
     *
     * @example
     * declare module 'express-session' {
     *     interface SessionData {
     *         views: number;
     *     }
     * }
     *
     */
    interface SessionData {
        cookie: Cookie;
    }

So you should declare the user properties on your session object like this:

declare module 'express-session' {
  interface SessionData {
    user: string;
  }
}

app.post('/login', express.urlencoded({ extended: false }), function (req, res) {
  req.session.user = req.body.user
})

package version: "express-session": "^1.17.1"