AngularJS $q.all & multiple $q.defer

Question

Even though I have managed to make my code work, there is something I don't understand. The following piece of code functions correctly:

socket.on('method', function() {
    var payload = {
      countrycode: '',
      device: ''
    };
    var d1 = $q.defer();
    var d2 = $q.defer();
    $q.all([
      geolocation.getLocation().then(function(position) {
        geolocation.getCountryCode(position).then(function(countryCode){
          payload.countrycode = countryCode;
          d1.resolve(countryCode);
        });
        return d1.promise;
      }),
      useragent.getUserAgent().then(function(ua) {
        useragent.getIcon(ua).then(function(device) {
          payload.device =  device;
          d2.resolve(device);
        });
        return d2.promise
      })
    ]).then(function(data){
      console.log(data); //displays ['value1', 'value2']
    })
  });

Is there a better way of achieving this? Before I had only one deferred variable, i.e. varvar deferred = $q.defer(); but that way the .then() function returned an object with double the results.

So the few question I have are:

1. Do I need multiple $q.defer vars?
2. Is the above the best way to wait for two async calls to finish and populate the payload object?

Answer 1

socket.on('method', function() {
    var payload = {
      countrycode: '',
      device: ''
    };
    geolocation.getLocation()
    .then(function(position) {
      return geolocation.getCountryCode(position);
    })
    .then(function(countryCode) {
      payload.countrycode = countryCode;
      return useragent.getUserAgent();
    })
    .then(function(ua) {
      return useragent.getIcon(ua);
    })
    .then(function(device) {
      payload.device =  device;
      console.log(data); //displays ['value1', 'value2']
    });
});

read the promise chaining part

Answer 2

You could always separate your code into smaller semantic blocks like so:

getCountryCode = function() {
  var d = $q.defer();
  geolocation.getLocation()
  .then(function(position) {
    return geolocation.getCountryCode(position)
  })
  .then(function(countryCode) {
    d.resolve(countryCode);
  })
  .fail(function(err) {
    d.reject(err);
  })
  return d.promise;
};

getDevice = function() {
  var d = $q.defer();
  useragent.getUserAgent()
  .then(function(ua) {
    return useragent.getIcon(ua)
  })
  .then(function(device) {
    d.resolve(device);
  })
  .fail(function(err) {
    d.reject(err);
  });
  return d.promise;
}

That will shorten your actual parallel call ($q.all) quite a bit:

socket.on('method', function() {
  $q.all([getCountryCode(), getDevice()])
    .spread(function(countryCode, device) {
      var payload = {
        countryCode: countryCode,
        device: device
      };
      // ... do something with that payload ...
    });
});

Answer 3

To synchronize multiple asynchronous functions and avoid Javascript callback hell: http://fdietz.github.io/recipes-with-angular-js/consuming-external-services/deferred-and-promise.html