Javascript: return nested function(asynchronus)

Question

I want to access(or print) d from return of function getDistanceFromLatLonInKm just by calling get.Location .I guess I may use callbacks but it dosent't work(and/or I'm a n00b).Is it possible to do this in this case? For example :document.write(getLocation(45.123,12.123)) //would print d somehow .

Thank you in advance.

getLocation(45.123,12.123);

function getLocation(a,b) {
  if (navigator.geolocation) {
    navigator.geolocation.getCurrentPosition(function(p){ajmo(p,a,b);});
  }
}

function ajmo(position,a,b) {
  lat = position.coords.latitude;
  lng = position.coords.longitude;
  getDistanceFromLatLonInKm(a,b,lat, lng);
}


function getDistanceFromLatLonInKm(lat_origin, lon_origin, lat_pos, lon_pos) {
  var R = 6371;
  var dLat = deg2rad(lat_pos - lat_origin);
  var dLon = deg2rad(lon_pos - lon_origin);
  var a = 
    Math.sin(dLat / 2) * Math.sin(dLat / 2) +
    Math.cos(deg2rad(lat_origin)) * Math.cos(deg2rad(lat_pos)) * 
    Math.sin(dLon / 2) * Math.sin(dLon / 2)
    ; 
  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
  var d = R * c;
  return d;
}

function deg2rad(deg) {
  return deg * (Math.PI/180)
}

You don't return values from asynchronous code as you're working in CPS, you need callbacks, or promises. — elclanrs
– elclanrs, Commented May 6, 2014 at 22:15
Yup, return is crucial here and it's missing in first two functions. — ElmoVanKielmo
– ElmoVanKielmo, Commented May 6, 2014 at 22:17
possible duplicate of Javascript: access return of the nested function -- please don't ask questions twice (or thrice). — Felix Kling
– Felix Kling, Commented May 6, 2014 at 22:55

TasDiam · Accepted Answer · 2014-05-06 23:18:28Z

1

Assuming you have an HTML element with id = "distance", you probably need this:

function getLocation(a, b, element) {
  if (navigator.geolocation) {
    navigator.geolocation.getCurrentPosition(function (position) {
      lat = position.coords.latitude;
      lng = position.coords.longitude;
      var d = getDistanceFromLatLonInKm(a,b,lat, lng);
      document.getElementById(element).innerHTML = d;
    });
  }
}


function getDistanceFromLatLonInKm(lat_origin, lon_origin, lat_pos, lon_pos) {
  var R = 6371;
  var dLat = deg2rad(lat_pos - lat_origin);
  var dLon = deg2rad(lon_pos - lon_origin);
  var a = 
    Math.sin(dLat / 2) * Math.sin(dLat / 2) +
    Math.cos(deg2rad(lat_origin)) * Math.cos(deg2rad(lat_pos)) * 
    Math.sin(dLon / 2) * Math.sin(dLon / 2)
    ; 
  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
  var d = R * c;
  return d;
}

function deg2rad(deg) {
  return deg * (Math.PI/180)
}  


getLocation(45.123, 12.123, 'distance');

edited May 6, 2014 at 23:18

answered May 6, 2014 at 22:43

TasDiam

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7 Comments

TasDiam Over a year ago

Since navigator.geolocation.getCurrentPosition is asynchronous, getLocation function cannot return any outcome from the asynchronous operation.

Axon Over a year ago

So,it is impossible to write 'd' with document.write(getLocation(a,b));?

TasDiam Over a year ago

If you want to set 'd' to the outcome value, then you should declare it outside getDistanceFromLatLonInKm function. But, since d will be assigned asynchronously, it's better to think what you want to do with this value, rather than just get the value. If you want to print it, then above code is enough. If you want to access it, call the function you want after it is assigned.

Axon Over a year ago

I just want to print it(d) in html only by calling getLocation.

TasDiam Over a year ago

You are actually do that :) Just replace console.log with code for setting HTML to an element. If you want, you can even pass the element id or class to getLocation and set the HTML of a specific element. When working with asynchronous programming, you have to swift your mind from the notion of having a function return a value.

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