Node.js, Vue.js and Passport.js. .isAuthenticated() always returns false? Axios headers possibly?

Your issue is because your frontend and backend are on different domains.

Cookies, which passport.session()/express.session() use to maintain a user session, are scoped to a specific domain.

When you call axios.get() on the protected resource, axios will not send or receive cookies because localhost:8080 is a different domain to localhost:8081.

Try axios.get('/path/to/your/resource', { withCredentials: true }) and axios.post('/login', { username, password }, { withCredentials: true })

This would have been present even without Vue so long as you're using AJAX to make these calls.

requests.js

let axiosConfig = {
  withCredentials: true,
  headers: {
    'Content-Type': 'application/json',
    'Access-Control-Allow-Origin': 'http://localhost:3000/',
    'Access-Control-Allow-Methods': 'GET,PUT,POST,DELETE'
  }
}

server.js

var express  = require('express');
var app      = express();
var cors = require('cors');
app.use(cors({credentials: true, origin: 'http://localhost:8080'}))

app.use(cors({credentials: true, origin: 'http://localhost:8080'}))