feat($q): add $q.always() method

Add $q.always(callback) method that is always called whether the promise is successful or fails; includes unit tests and updates
documentation.

src/ng/q.js

@@ -91,6 +91,11 @@
  *   This method *returns a new promise* which is resolved or rejected via the return value of the
  *   `successCallback` or `errorCallback`.
  *
+ * - `always(callback)` – allows you to observe either the fulfillment or rejection of a promise,
+ *   but to do so without modifying the final value. This is useful to release resources or do some
+ *   clean-up that needs to be done whether the promise was rejected or resolved. See the [full
+ *   specification](https://github.com/kriskowal/q/wiki/API-Reference#promisefinallycallback) for
+ *   more information.
  *
  * # Chaining promises
  *
@@ -236,6 +241,42 @@ function qFactory(nextTick, exceptionHandler) {
           }
 
           return result.promise;
+        },
+        always: function(callback) {
+
+          function makePromise(value, resolved) {
+            var result = defer();
+            if (resolved) {
+              result.resolve(value);
+            } else {
+              result.reject(value);
+            }
+            return result.promise;
+          }
+
+          function handleCallback(value, isResolved) {
+            var callbackOutput = null;            
+            try {
+              callbackOutput = (callback ||defaultCallback)();
+            } catch(e) {
+              return makePromise(e, false);
+            }            
+            if (callbackOutput && callbackOutput.then) {
+              return callbackOutput.then(function() {
+                return makePromise(value, isResolved);
+              }, function(error) {
+                return makePromise(error, false);
+              });
+            } else {
+              return makePromise(value, isResolved);
+            }
+          }
+
+          return this.then(function(value) {
+            return handleCallback(value, true);
+          }, function(error) {
+            return handleCallback(error, false);
+          });
         }
       }
     };

test/ng/qSpec.js

@@ -345,6 +345,10 @@ describe('q', function() {
       it('should have a then method', function() {
         expect(typeof promise.then).toBe('function');
       });
+
+      it('should have a always method', function() {
+        expect(typeof promise.always).toBe('function');
+      });
 
 
       describe('then', function() {
@@ -461,6 +465,134 @@ describe('q', function() {
           expect(log).toEqual(['error(oops!)']);
         });
       });
+
+
+      describe('always', function() {
+
+        it('should not take an argument',
+            function() {
+          promise.always(success(1))
+          syncResolve(deferred, 'foo');
+          expect(logStr()).toBe('success1()');
+        });
+
+        describe("when the promise is fulfilled", function () {
+
+          it('should call the callback',
+              function() {
+            promise.then(success(1))
+                   .always(success(2))
+            syncResolve(deferred, 'foo');
+            expect(logStr()).toBe('success1(foo); success2()');
+          });
+
+          it('should fulfill with the original value',
+              function() {
+            promise.always(success(1))
+                   .then(success(2), error(2))
+            syncResolve(deferred, 'foo');
+            expect(logStr()).toBe('success1(); success2(foo)');
+          });
+
+          describe("when the callback returns a promise", function() {
+
+            describe("that is fulfilled", function() {
+              it("should fulfill with the original reason after that promise resolves",
+                function () {
+                var returnedDef = defer()
+                returnedDef.resolve('bar');
+                promise.always(success(1, returnedDef.promise))
+                       .then(success(2))
+                syncResolve(deferred, 'foo');
+                expect(logStr()).toBe('success1(); success2(foo)');
+              });
+            });
+
+            describe("that is rejected", function() {
+              it("should reject with this new rejection reason",
+                function () {
+                var returnedDef = defer()
+                returnedDef.reject('bar');
+                promise.always(success(1, returnedDef.promise))
+                       .then(success(2), error(1))
+                syncResolve(deferred, 'foo');
+                expect(logStr()).toBe('success1(); error1(bar)');
+              });
+            });
+
+          });
+
+          describe("when the callback throws an exception", function() {
+            it("should reject with this new exception", function() {
+              promise.always(error(1, "exception", true))
+                     .then(success(1), error(2))
+              syncResolve(deferred, 'foo');
+              expect(logStr()).toBe('error1(); error2(exception)');
+            });
+          });
+
+        });
+
+
+        describe("when the promise is rejected", function () {
+
+          it("should call the callback", function () {
+            promise.always(success(1))
+                   .then(success(2), error(1))
+            syncReject(deferred, 'foo');
+            expect(logStr()).toBe('success1(); error1(foo)');
+          });
+
+          it('should reject with the original reason', function() {
+            promise.always(success(1), "hello")
+                   .then(success(2), error(2))
+            syncReject(deferred, 'original');
+            expect(logStr()).toBe('success1(); error2(original)');
+          });
+
+          describe("when the callback returns a promise", function() {
+
+            describe("that is fulfilled", function() {
+
+              it("should reject with the original reason after that promise resolves", function () {
+                var returnedDef = defer()
+                returnedDef.resolve('bar');
+                promise.always(success(1, returnedDef.promise))
+                       .then(success(2), error(2))
+                syncReject(deferred, 'original');
+                expect(logStr()).toBe('success1(); error2(original)');
+              });
+
+            });
+
+            describe("that is rejected", function () {
+
+              it("should reject with the new reason", function() {
+                var returnedDef = defer()
+                returnedDef.reject('bar');
+                promise.always(success(1, returnedDef.promise))
+                       .then(success(2), error(1))
+                syncResolve(deferred, 'foo');
+                expect(logStr()).toBe('success1(); error1(bar)');
+              });
+
+            });
+
+          });
+
+          describe("when the callback throws an exception", function() {
+
+            it("should reject with this new exception", function() {
+              promise.always(error(1, "exception", true))
+                     .then(success(1), error(2))
+              syncResolve(deferred, 'foo');
+              expect(logStr()).toBe('error1(); error2(exception)');
+            });
+
+          });
+
+        });        
+      });
     });
   });