即使我声明了会话，Express-session也会返回“ undefined”

Question

I've been working on this website for a while and didn't really have a problem, 'till now.

When the user logs in, a session cookie is initialized this cookie is called user and stores the users email. When I console log the cookie on the login post request, just after I declared it, it shows data, but, when I change of route (to any other) ex. the home route, the cookie is not persistent and my user cookie changes to 'undefined'.

Post Request, I'm using the firebase API for authentification:

// Login POST Route
router.post('/login', (req, res) => {
    // Firebase authentication service
    firebase_user.auth().signInWithEmailAndPassword(req.body.email, req.body.password).then(data => {
        // Cookie Init
        req.session.user = req.body.email;
        console.log(req.session.user); // In here, cookie shows desired value
    }).catch(err => {
        res.send({"error": err.message});
    });
});

Home route:

router.get('/home', (req, res) => {
    // Check if Session Cookie Exists
    if (req.session.user) {
        res.render('home.ejs');
    } else {
        res.redirect('/login');
        console.log(req.session.user); // This console log shows 'undefined' even tho there the cookie was initialized correctly
    }
});

Middlewares:

app.use(bodyParser.json());
app.use(morgan('combined'));
app.set('view engine', 'ejs');
app.use(express.static('./public'))
app.set('views', path.join(__dirname, 'views'));
app.use(bodyParser.urlencoded({ extended: true }));
app.use(cookieParser());
app.use(session({secret:"Testasl",resave:false,saveUninitialized:true,cookie:{secure:!true}}));

// Routes
app.use(routes);

And here is how I send the data to the login method. I use Axios and Vue:

var urlLog = 'http://localhost:3000/login';

new Vue({
    el: '#main',
    data: {
        email: '',
        password: '',
        showForm: true,
        showPreloader: false,
        errorMessage: '',
        errorShow: false

    },
    methods: {
        submitForm: function() {
            // Validates forms
            if (this.email!='' && this.password!=''){

                // Show Preloader
                this.showForm=false;
                this.showPreloader=true;

                // Ajax Post Request
                axios.post(urlLog, {
                    email: this.email,
                    password: this.password
                }).then(res => {
                    if (res.error){
                        // Shows form
                        this.showForm=true;
                        this.showPreloader=false;

                        // Shows Error
                        this.errorShow = true;
                        this.errorMessage = res.error;
                    } else {
                        // do nothing
                    }
                // Server Side error
                }).catch(err => {
                    console.log(err);
                });
            } else {
                this.errorShow = true;
                this.errorMessage = 'All fields are necessary...';   
            }
        }
    }
});

Any idea why this is happening???

**** EDITED ****

UPDATE: So I was playing around with cookies, to be precise, with the cookie-parser module, and I decided to initialize a cookie with it. And returned this error message:

Error: Can't set headers after they are sent.
    at validateHeader (_http_outgoing.js:491:11)
    at ServerResponse.setHeader (_http_outgoing.js:498:3)
    at ServerResponse.header (C:\Users\Thirsty-Robot\Desktop\Projects\Important\Robotics\Dashboard\node_modules\express\lib\response.js:767:10)
    at ServerResponse.append (C:\Users\Thirsty-Robot\Desktop\Projects\Important\Robotics\Dashboard\node_modules\express\lib\response.js:728:15)
    at ServerResponse.res.cookie (C:\Users\Thirsty-Robot\Desktop\Projects\Important\Robotics\Dashboard\node_modules\express\lib\response.js:853:8)
    at router.get (C:\Users\Thirsty-Robot\Desktop\Projects\Important\Robotics\Dashboard\bin\routes.js:74:9)
    at Layer.handle [as handle_request] (C:\Users\Thirsty-Robot\Desktop\Projects\Important\Robotics\Dashboard\node_modules\express\lib\router\layer.js:95:5)
    at next (C:\Users\Thirsty-Robot\Desktop\Projects\Important\Robotics\Dashboard\node_modules\express\lib\router\route.js:137:13)
    at Route.dispatch (C:\Users\Thirsty-Robot\Desktop\Projects\Important\Robotics\Dashboard\node_modules\express\lib\router\route.js:112:3)
    at Layer.handle [as handle_request] (C:\Users\Thirsty-Robot\Desktop\Projects\Important\Robotics\Dashboard\node_modules\express\lib\router\layer.js:95:5)

The cookie was set in this way:

// Login GET Route
router.get('/login', (req, res) => {
    res.render('log_in.ejs');
    res.cookie('idk', 'idksj');
    console.log(req.cookies);
});

Answer 1

You can't set a header after already sending the headers, as your error states.

In your case, you're setting the cookie header after ending the response:

// Login GET Route
router.get('/login', (req, res) => {
    res.render('log_in.ejs'); // completes the response
    res.cookie('idk', 'idksj'); // tries to set a cookie header
    console.log(req.cookies);
});

For this case, you would be well served to just swap those two lines:

// Login GET Route
router.get('/login', (req, res) => {
    res.cookie('idk', 'idksj');
    res.render('log_in.ejs');
    console.log(req.cookies);
});

Though you probably want to do that only after validating the login (which you're not really doing in this code block)