attempt-3.js

const request = require('request').defaults({ jar: true});

const LOCATIONS = { /* ... */ };
const PAGE_REGEXP = /(\d+-\d+)\.IBehaviorListener/;
const URL_ROOT = 'https://www.migrationsverket.se/ansokanbokning/';

function getAppointmentsForLocation (location) {
    const bookingPageUrl = `valjtyp?sprak=en&bokningstyp=2&enhet=${LOCATIONS[location]}&sokande=1`;
    request(`${URL_ROOT}${bookingPageUrl}`, (_, _, body) => {
        
        const [, page] = body.match(PAGE_REGEXP);
        // ... Date stuff
        const appointmentsUrl = `wicket/page?${page}.IBehaviorListener.1-form-kalender-kalender&start=${start}&end=${end}`;
        request(appointmentsUrl, (_, _, body) => {

            const appointments = JSON.parse(body);
            appointments.forEach(appointment => {
                console.log(`🇸🇪💉 Available appointment in ${location} at ${new Date(appointment.start)}!`);
            });
        });
    });
}

getAppointmentsForLocation(LOCATIONS.STOCKHOLM);