The way I managed to get this to work was by the code below:

const express = require('express');
const SpotifyWebApi = require('spotify-web-api-node');

var generateRandomString = function(length) {
    var text = '';
    var possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';

    for (var i = 0; i < length; i++) {
        text += possible.charAt(Math.floor(Math.random() * possible.length));
    }

    return text;
};

var scopes = ['user-read-private', 'user-read-email', 'playlist-read-private', 'playlist-modify-private'],
    redirectUri = '<redirect uri>',
    clientId = '<client id>',
    clientSecret = '<client secret>',
    state = generateRandomString(16);

// Setting credentials can be done in the wrapper's constructor, or using the API object's setters.
var spotifyApi = new SpotifyWebApi({
    redirectUri: redirectUri,
    clientId: clientId,
    clientSecret: clientSecret
});

// Create the authorization URL
var authorizeURL = spotifyApi.createAuthorizeURL(scopes, state);

// https://accounts.spotify.com:443/authorize?client_id=5fe01282e44241328a84e7c5cc169165&response_type=code&redirect_uri=https://example.com/callback&scope=user-read-private%20user-read-email&state=some-state-of-my-choice
console.log(authorizeURL);

// --------------------------------

var credentials = {
    clientId: '<client id>',
    clientSecret: '<client secret>',
    redirectUri: '<redirect uri>'
};

var spotifyApi = new SpotifyWebApi(credentials);

var app = express();

app.get('/login', function(req, res) {
    res.redirect(authorizeURL);
});

// The code that's returned as a query parameter to the redirect URI
var code = '<authorization code>'; // this does not need to be updated

// Retrieve an access token and a refresh token
spotifyApi.authorizationCodeGrant(code).then(
    function(data) {
        console.log('The token expires in ' + data.body['expires_in']);
        console.log('The access token is ' + data.body['access_token']);
        console.log('The refresh token is ' + data.body['refresh_token']);

        // Set the access token on the API object to use it in later calls
        spotifyApi.setAccessToken(data.body['access_token']);
        spotifyApi.setRefreshToken(data.body['refresh_token']);
    },
    function(err) {
        console.log('Something went wrong!', err);
    }
);
// --------------------------------------------------
// clientId, clientSecret and refreshToken has been set on the api object previous to this call.
function refreshSpotifyToken() {
    spotifyApi.refreshAccessToken().then(
        function(data) {
            console.log('The access token has been refreshed!');

            // Save the access token so that it's used in future calls
            spotifyApi.setAccessToken(data.body['access_token']);
            console.log('The access token is ' + data.body['access_token']);
            console.log('The token expires in ' + data.body['expires_in']);
        },
        function(err) {
            console.log('Could not refresh access token', err);
        });
};
client.on('ready', () => {
    refreshSpotifyToken();
    setInterval(refreshSpotifyToken, 1000 * 59 * 59);
})

Information on best practice Auth Flow directly from Spotify can be found at the link below. It includes code snippets detailing how and when they want you to POST back the refresh token for a new Access Token.

https://developer.spotify.com/documentation/general/guides/authorization/code-flow/

This is the endpoint they recommend for refreshing your token when you detect an Access Token has expired or your Request for data failed due to expired Access Token.

app.get('/refresh_token', function(req, res) {

  var refresh_token = req.query.refresh_token;
  var authOptions = {
    url: 'https://accounts.spotify.com/api/token',
    headers: { 'Authorization': 'Basic ' + (new Buffer(client_id + ':' + client_secret).toString('base64')) },
    form: {
      grant_type: 'refresh_token',
      refresh_token: refresh_token
    },
    json: true
  };

  request.post(authOptions, function(error, response, body) {
    if (!error && response.statusCode === 200) {
      var access_token = body.access_token;
      res.send({
        'access_token': access_token
      });
    }
  });
});

The response will look something like this:

{
   "access_token": "NgCXRK...MzYjw",
   "token_type": "Bearer",
   "scope": "user-read-private user-read-email",
   "expires_in": 3600,
   "refresh_token": "NgAagA...Um_SHo"
}

TLDR: You check to see if your Access Token has expired, if so POST the refresh-token to receive a new Access Token that can then again be used to GET data. You also receive a new refresh token along with the response from this and the process starts all over again.