Question

I use this code to calculate to distance between two gps positions. Problem is when i return calculated value it returns undefined value. Please help me

function calcDistane(offerID,userLocation){
    var dist;
    var adapter = new LocationAdapter();

    adapter.selectByOfferId(offerID,function(res){                       

    navigator.geolocation.getCurrentPosition(function(position){

        var R = 6371;
        var userLocation= position.coords;

        dist= Math.acos(Math.sin(userLocation.latitude)*Math.sin(res.item(0).lt) + 
              Math.cos(userLocation.latitude)*Math.cos(res.item(0).lt) *
              Math.cos(userLocation.longitude-res.item(0).lg)) * R;
        });

    });  
    return dist;
};

Answer 1

dist isn’t set yet when you return. The function that is setting dist is a callback. That is very likely to be called after you return from the outer (callback) function.

The likely order of execution is

1. adapter.selectByOfferId
2. return dist (undefined)
3. call anonymous function used as callback to adapter.selectByOfferId
4. Call navigator.geolocation.getCurrentPosition and return from the callback from step 3
5. When navigator.geolocation.getCurrentPosition returns then the callback of that call is called and dist is set. Which is after step 2

You will need to pass a continuation rather than returning

function calcDistane(offerID,userLocation,callback){
  var adapter = new LocationAdapter();

  adapter.selectByOfferId(offerID,function(res){                       

        navigator.geolocation.getCurrentPosition(function(position){

               var R = 6371;
               var userLocation= position.coords;

               callback(Math.acos(Math.sin(userLocation.latitude)*Math.sin(res.item(0).lt) + 
                        Math.cos(userLocation.latitude)*Math.cos(res.item(0).lt) *
                        Math.cos(userLocation.longitude-res.item(0).lg)) * R);

       });

  });
}