Will there be any inconsistency with the value calculated?

Question

I have a javascript code like below to calculate the total distance between n markers.

var distance = 0;
function updateTimeAndDistance(timeAndPath) {
    realtracPath = timeAndPath.path;
    getDistance();
   console.log("calculated distance : " + distance);
}

function getDistance() {

    for ( var i = 0; i < realtracPath.length - 1 ; i++) {
        var startPos = new google.maps.LatLng(realtracPath[i].lat, realtracPath[i].lng);
        var endPos = new google.maps.LatLng(realtracPath[i+1].lat, realtracPath[i+1].lng);
        var request = {
            origin : startPos,
            destination : endPos,
            travelMode : google.maps.TravelMode.DRIVING
        };
        directionsService.route(request, function(response, status) {
            if (status == google.maps.DirectionsStatus.OK) {
                distance += response.routes[0].legs[0].distance.value;
            }
        });


    }
}

but I am worried that, will there be any inconsistency in the value of the distance calculated as the distance is calculated asynchronously.

EDIT: every time I run this, I am getting distance as zero. I am not understanding why, though I have used the global distance variable.

Thanks.

Your variable realtrackPath does not exist on the global scope. I.e, my guess is that your loop is never entered. Try adding `var realtracPath;` to the top of your code. Note: it is probably better to be passing these variables around rather than leaving them outside the scope of your functions. — Joe, Jun 30 '14 at 15:41
Looks like a duplicate of [How to return the response from an Ajax call?](http://stackoverflow.com/questions/14220321/how-to-return-the-response-from-an-ajax-call) — apsillers, Jun 30 '14 at 15:43
@ joe, all the variables are defined as per the scope. Sorry for not uploading the whole code. — GuruKulki, Jun 30 '14 at 15:44
@Joe This is probably a timing issue, not a scope issue (i.e., `console.log` runs chronologically before the `directionsService.route` callback function). — apsillers, Jun 30 '14 at 15:46
@apsillers, Yeah it is similar to the question you mentioned, but I am not getting any clue how to to use the answer provided there in my case. — GuruKulki, Jun 30 '14 at 15:50
cant you add a callback to the getDistance function `getDistance(cb)` and then call `cb()` after you have added the response to the `distance` variable? Then you should just call `getDistance(function(){ console.log(distance) })` — Joe, Jun 30 '14 at 15:50
@joe, I am not aware how to do that, as this I working on javascripts recently. — GuruKulki, Jun 30 '14 at 15:52
@joe, It is working now, thank you very much. Now I understand how the callback feature works in javascript. — GuruKulki, Jun 30 '14 at 16:02
@joe, but there is one small prob, the callback gets called each time the loop runs, cant we avoid this to make it run only once? — GuruKulki, Jun 30 '14 at 16:07
Updated answer to include a latch. Should only callback when all loops have completed. Glad you get callbacks now. I must admit that coming from a java background the idea of passing functions took me a while to get my head around. It can be really powerful once you get to grips with it! — Joe, Jun 30 '14 at 16:41

Joe · Accepted Answer · 2014-06-30T16:41:01.110

Try introducing an async callback function like so;

var distance = 0;
function updateTimeAndDistance(timeAndPath) {
    realtracPath = timeAndPath.path;
    getDistance(function(){
        console.log("calculated distance : " + distance);
    });

}

function getDistance(cb) {
var latch = realtrackPath.length;
for ( var i = 0; i < realtracPath.length - 1 ; i++) {
    var startPos = new google.maps.LatLng(realtracPath[i].lat, realtracPath[i].lng);
    var endPos = new google.maps.LatLng(realtracPath[i+1].lat, realtracPath[i+1].lng);
    var request = {
        origin : startPos,
        destination : endPos,
        travelMode : google.maps.TravelMode.DRIVING
    };
    directionsService.route(request, function(response, status) {
        if (status == google.maps.DirectionsStatus.OK) {
            distance += response.routes[0].legs[0].distance.value;
            latch--
            if (latch==0){
               cb()
            }                
        }
    });


}

}

Note: If the status does not come back as OK nothing will happen. It can be quite handy to have callbacks with function signatures that pass booleans indicating errors etc.

Will there be any inconsistency with the value calculated?

1 Answers1