Decorator to add 'transform' and 'transformError' methods to $q promise chain.

qPromiseTransformDecorator.js

/**
 * Decorator to add 'transform' and 'transformError' methods to promises.
 * These can manipulate the result without catching errors so the 'then' 
 * functions will still work as expected.
 */
$provide.decorator('$q', function($delegate) {
  var defer = $delegate.defer;
  $delegate.defer = function() {
    var deferred = defer();
    deferred.promise.transform = function(fn) {
      var defer = $delegate.defer();
      deferred.promise.then(function(value) {
        defer.resolve(fn(value));
      }, function(value) {
        defer.reject(value);
      });
      return defer.promise;
    };
    deferred.promise.transformError = function(fn) {
      var defer = $delegate.defer();
      deferred.promise.then(function(value) {
        defer.resolve(value);
      }, function(value) {
        defer.reject(fn(value));
      });
      return defer.promise;
    };
    return deferred;
  };
  return $delegate;
});

/**
 * Usage example:
 */
var promise = (function() {
  var defer = $q.defer();
  $timeout(function() {
    defer.resolve(6);
  }, 1000);
  return defer.promise;
})();

promise
  .transform(function(data) {
    return data + 2;
  })
  .transform(function(data) {
    return data * 2;
  })
  .transformError(function(data) {
    return data / 2;
  })
  .transformError(function(data) {
    return data + 2;
  })
  .then(function(data) {
    console.log(data); // 16 on resolve
  }, function(data) {
    console.log(data); // 5 on reject
  });