Handling errors in multiple calls of AngularJS promises

Question

I have an AngularJS service with such async API:

myService.asyncCall(['id0', 'id3', 'id2']).then(function (complexData) {   
  // handle complexData
}).catch(function (error) {   
  console.log(error); 
});

asyncCall incapsulates multiple calls of $http which handled by $q.all. Each of the $http requests can response with error and I want the errors to be handled by one catch handler. So how i can achieve multiple calls of the catch handler ?

Answer 1

If I understood correctly (finally), this is what you are trying to achieve:

• Input:
  1. a list of promises
  2. a callback to be called when all promises have been resolved
  3. a callback to be called each time any of the promises gets rejected

---

• Behaviour:
  1. You want parallel execution (not sequential).
  2. You want all promises to be either resolved or rejected (i.e. even if one promise gets rejected, the rest should continue as usual).
  3. You want the "final" callback to be called exactly once (receiving an array of results - regardless if the respective deferred was resolved or rejected).

---

$q.all should be "augmented" to suit your needs, because by default:

1. It gets immediatelly rejected as soon as any of the promises in the list gets rejected.
2. In case of rejection it returns only the rejection reason and not a list of results.

---

Here is a possible implementation:

function asyncCall(listOfPromises, onErrorCallback, finalCallback) {

  listOfPromises  = listOfPromises  || [];
  onErrorCallback = onErrorCallback || angular.noop;
  finalCallback   = finalCallback   || angular.noop;

  // Create a new list of promises that can "recover" from rejection
  var newListOfPromises = listOfPromises.map(function (promise) {
    return promise.catch(function (reason) {

      // First call the `onErrroCallback`
      onErrorCallback(reason);

      // Change the returned value to indicate that it was rejected
      // Based on the type of `reason` you might need to change this
      // (e.g. if `reason` is an object, add a `rejected` property)
      return 'rejected:' + reason;

    });
  });

  // Finally, we create a "collective" promise that calls `finalCallback` when resolved.
  // Thanks to our modifications, it will never get rejected !
  $q.all(newListOfPromises).then(finalCallback);
}

See, also, this short demo.