I am trying to authenticate Spotify API using the OAuth2.0 client credentials flow. Generally the flow is as follows:

1. Send GET request to /authenticate endpoint with required parameters, including a callback endpoint that i have set to my http://localhost:8888/callback

const open = require('open')

responseType = 'token'
clientID = 'CLIENT_ID'
redirectURI = 'http%3A%2F%2Flocalhost%3A8888%2Fcallback'
scope = 'streaming+user-read-email+user-modify-playback-state+user-read-private+user-library-read+user-library-modify+user-read-currently-playing'
contentType = 'application/json'

params = {
  'response_type' : responseType,
  'client_id' : clientID,
  'redirect_uri' : redirectURI,
  'scope' : scope
}

headers = {
  'Content-Type' : contentType
}

// send user to browser to grant data permissions
open(`https://accounts.spotify.com/en/authorize?response_type=${responseType}&client_id=${clientID}&redirect_uri=${redirectURI}&scope=${scope}&show_dialog=true`)

-- this part is fine, i have no issues sending user to browser to login and grant my app permission and the Express server i have listening on port 8888 lets me know when the callback pings with the response

2. Receive Authentication Token in params of callback from Authentication endpoint. Spotify API handles this part and sends the authentication token that i need as part of the header in the response. it looks like this http://localhost:8888/callback#authentication_token=2iais820zg...fh9IHkLDI&timeout=3600 -- this is what i am having problems with.

here is my how i am handling the api callback in my Express server's /callback endpoint:

app.get('/callback', function (req, res) {
    console.log('someone made a request!\n')
    ///                             ////
    console.log(req + '\n')
    console.log(res + '\n')
    ///                             ////
    console.log(req.body)
    res.json(req.body)
    res.send('GET request to callback endpoint took you here')
    res.end()
})

but the console output is as follows:

> server is listening on port 8888 ...
someone made a request!

[object Object]

[object Object]

{}

where everything is empty.

how can i access the URL parameters of the callback GET request? i have tried changing the handling from GET to POST as people have told me that a GET request doesn't contain params but the callback can't find the endpoint (bc it's not making a POST request..)