Javascript - how to work with the iterator in a for loop with callbacks

Question

I am trying in the for loop to access the value of the i with which the callback function uses.

How can I do this?

for (var i = 0; i < a.length; i++)
{
    calcRoute(fixedLocation, my_cities[i].address, function(response) {

        // i want here to have the current "i" here

    });             
}

which calls...

function calcRoute(x, y, callback) {

    var start = x;
    var end = y;

    var request = {
        origin:start,
        destination:end,
        travelMode: google.maps.TravelMode.DRIVING,
        unitSystem: google.maps.UnitSystem.METRIC,
        optimizeWaypoints: true
    };

    directionsService.route(request, function(response, status) {
        if (status == google.maps.DirectionsStatus.OK) {
            callback(response);                                                                 
        } else {
            alert("City unknown.")
        }       
    }); 
}

Answer 1

It's because the closure captures the variable i itself, not the current value. Try:

for (var i = 0; i < a.length; i++) (function(i)
{
    calcRoute(fixedLocation, my_cities[i].address, function(response) {

        // i want here to have the current "i" here

    });             

}) (i);

which will create a new i variable for each loop iteration.