Add .lastArg and .callback to spy call

docs/release-source/release/spy-call.md

@@ -83,6 +83,36 @@ The call's `this` value.
 Array of received arguments.
 
 
+### `spyCall.callback`
+
+This property is a convenience for a call's callback.
+
+When the last argument in a call is a `Function`, then `callback` will reference that. Otherwise it will be `undefined`.
+
+```js
+var spy = sinon.spy();
+var callback = function () {};
+
+spy(1, 2, 3, callback);
+
+spy.lastCall.callback === callback;
+// true
+```
+
+#### `spyCall.lastArg`
+
+This property is a convenience for the last argument of the call.
+
+```js
+var spy = sinon.spy();
+var date = new Date();
+
+spy(1, 2, date);
+
+spy.lastCall.lastArg === date;
+// true
+```
+
 ### `spyCall.exception`
 
 Exception thrown, if any.

lib/sinon/call.js

@@ -222,10 +222,16 @@ function createSpyCall(spy, thisValue, args, returnValue, exception, id, errorWi
     if (typeof id !== "number") {
         throw new TypeError("Call id is not a number");
     }
+
     var proxyCall = Object.create(callProto);
+    var lastArg = args.length > 0 && args[args.length - 1] || undefined;
+    var callback = lastArg && typeof lastArg === "function" ? lastArg : undefined;
+
     proxyCall.proxy = spy;
     proxyCall.thisValue = thisValue;
     proxyCall.args = args;
+    proxyCall.lastArg = lastArg;
+    proxyCall.callback = callback;
     proxyCall.returnValue = returnValue;
     proxyCall.exception = exception;
     proxyCall.callId = id;

test/call-test.js

@@ -435,6 +435,41 @@ describe("sinonSpy.call", function () {
         });
     });
 
+    describe(".callback", function () {
+        it("it should be a reference for the callback", function () {
+            var spy = sinonSpy();
+            var callback1 = function () {};
+            var callback2 = function () {};
+
+            spy(1, 2, 3, callback1);
+            assert.equals(spy.getCall(0).callback, callback1);
+
+            spy(1, 2, 3, callback2);
+            assert.equals(spy.getCall(1).callback, callback2);
+
+            spy(1, 2, 3);
+            assert.equals(spy.getCall(2).callback, undefined);
+        });
+    });
+
+    describe(".lastArg", function () {
+        it("should be the last argument from the call", function () {
+            var spy = sinonSpy();
+
+            spy(41, 42, 43);
+            assert.equals(spy.getCall(0).lastArg, 43);
+
+            spy(44, 45);
+            assert.equals(spy.getCall(1).lastArg, 45);
+
+            spy(46);
+            assert.equals(spy.getCall(2).lastArg, 46);
+
+            spy();
+            assert.equals(spy.getCall(3).lastArg, undefined);
+        });
+    });
+
     describe("call.yieldTest", function () {
         beforeEach(spyCallCallSetup);