Iterating with callback/anonymous functions

Question

I'm new to Node.JS and advanced Javascript in general, but I'm trying to build a schedule manager application on my own and I faced a problem (I will detail it later) when trying to execute the following code:

router.get('/', function (req, res) {
var day = new Date(req.query.day);
Location.getLocations(function (err, locations) {
    if (locations.length > 0) {
        var i;
        for (i = 0; i < locations.length; i++) {
            var location = locations[i];
            Appointment.getAppointments(day, location, function (err, appointments) {
                if (err) throw err;
                if (appointments.length == 0) {
                    // CREATE APPOINTMENTS
                    for (var j = location.available_time_start; j <= location.available_time_end; j += location.appointment_duration) {
                        var newAppointment = new Appointment();

                        newAppointment.start_date = new Date(day.getFullYear(), day.getMonth() + 1, day.getDate(), j);
                        newAppointment.appointment_duration = location.appointment_duration;
                        newAppointment.location = location.id;
                        newAppointment.booked = false;
                        newAppointment.locked = false;

                        Appointment.createAppointment(newAppointment, function (err, appointment) {
                            if (err) throw err;
                            console.log(appointment.location + ' - ' + appointment.start_date);
                        });
                    }
                }
            });
        }
    } else {
        // THERE ARE NO LOCATIONS
    }

    res.render('appointments', { locations: locations });
});

The problem is:
When I try to iterate the locations object and then execute the getAppointments function the code isn't executed at this exact moment. Later, when it's executed, location object is always the same (the iteration doesn't work), resulting on a unexpected result (all appointments with the same/last location).

I tried using IIFE (Immediately-invoked function expression) to execute the code instantly, but when I did this I couldn't get the appointments callback object and my logic is broken too.

Thanks in advance!

Answer 1

The problem was solved by using let instead of var as suggested by @JaromandaX.