Just wondering if this is even possible, I believe it is by searching in the internet and I made my attempt from these sources.

airport_data = [{"departure_time" :"12:00","arrival_time":"03:00","city_id":"BOS"},
{"departure_time" :"01:00","arrival_time":"04:00","city_id":"SFO"},
{"departure_time" :"03:00","arrival_time":"05:00","city_id":"BOS"},
{"departure_time" :"03:00","arrival_time":"05:00","city_id":"SFO"},
{"departure_time" :"04:00","arrival_time":"06:00","city_id":"SJC"},
{"departure_time" :"04:00","arrival_time":"06:00","city_id":"JFK"},
{"departure_time" :"06:00","arrival_time":"09:00","city_id":"SJC"}];

for(var i = 0; i < airport_data.length; i++) {
    if (airport_data.city_idindexOf(city.id[i])){
        var counter_{airport_data.city_id} += 1; 
    }
}

Goal

counter_BOS = 2;
counter_SFO = 1;
counter_SJC = 2;
counter_JFK = 1;
counter_JFC = 1;