INDEPENDENT parallel promises with AngularJS $q.all

independent-qall-parallel-promises_1.js

/*
 * @license CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/)
 *
 * Angular's $q.all works great for cases where a rejection of any one promise
 * SHOULD cause all parallel promises to be rejected... but it's not so great
 * when you've got a number of promises that need to run in parallel and you
 * don't want a single rejection to reject the entire $q.all promise.
 * (One example: think aggregating data from multiple APIs.)
 *
 * This gist shows one way to "partially resolve" parallel processes when at
 * least one $q.all promise is rejected. To keep the code concise and focused,
 * just imagine that itGetsResolved.query() and itGetsRejected.query() return
 * $http.get promises that do what they suggest (get resolved and rejected,
 * respectively).
 */

var example = angular.module('independent-parallel-promises', [
  'it-gets-resolved-service',
  'it-gets-rejected-service'
]);

example.service('aggregator', ['$q', '$log', function($q, $log) {
  /** @private */
  function keepResponse(response) {
    return response;
  }

  /** @private */
  function logAndKeepError(e) {
    $log.error('rejected query', e.config.url);
    return e;
  }

  this.aggregate = function() {
    return $q(function(resolve, reject) {
      $q.all([
        itGetsResolved.query().then(keepResponse).catch(logAndKeepError),
        itGetsRejected.query().then(keepResponse).catch(logAndKeepError)
      ])
      .then(function(responses) {
        // it's up to YOU whether to resolve or reject the aggregate promise!
        resolve({
          'it-gets-resolved-response': responses[0], // array of whatever
          'it-gets-rejected-response': responses[1] // an Error object
        });
      });
      // NOTE: q.all.catch won't be invoked in this scenario
    });
  };
}]);

independent-qall-parallel-promises_2.js

/**
 * @license CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/)
 *
 * An alternative implementation of the 'aggregator' service that simply
 * uses an empty array instead of an Error object when a promise is rejected,
 * and resolves with a concatenated array of all responses.
 */

example.service('aggregator', ['$q', '$log', function($q, $log) {
  /** @private */
  function keepResponse(response) {
    return response;
  }

  /** @private */
  function logAndIgnoreError(e) {
    $log.error('ignoring rejected query', e.config.url);
    return [];
  }

  this.aggregate = function() {
    return $q(function(resolve, reject) {
      $q.all([
        itGetsResolved.query().then(keepResponse).catch(logAndIgnoreError),
        itGetsRejected.query().then(keepResponse).catch(logAndIgnoreError)
      ])
      .then(function(responses) {
        resolve(
          responses[0] // array of whatever
          .concat(responses[1]) // empty array
        );
      });
    });
  };
}]);