Add firstCall convenience property to get the first argument of the l…

…ast call of a fake.

docs/_releases/latest/fakes.md

@@ -130,6 +130,22 @@ f.lastCall.callback === cb2;
 // true
 ```
 
+#### `f.firstArg`
+
+This property is a convenient way to get a reference to the first argument passed in the last call to the fake.
+
+```js
+var f = sinon.fake();
+var date1 = new Date();
+var date2 = new Date();
+
+f(date1, 1, 2);
+f(date2, 1, 2);
+
+f.firstArg === date2;
+// true
+```
+
 #### `f.lastArg`
 
 This property is a convenient way to get a reference to the last argument passed in the last call to the fake.

lib/sinon/fake.js

@@ -14,9 +14,16 @@ var uuid = 0;
 function wrapFunc(f) {
     var proxy;
     var fakeInstance = function() {
-        var lastArg = arguments.length > 0 ? arguments[arguments.length - 1] : undefined;
+        var firstArg, lastArg;
+
+        if (arguments.length > 0) {
+            firstArg = arguments[0];
+            lastArg = arguments[arguments.length - 1];
+        }
+
         var callback = lastArg && typeof lastArg === "function" ? lastArg : undefined;
 
+        proxy.firstArg = firstArg;
         proxy.lastArg = lastArg;
         proxy.callback = callback;
 

test/fake-test.js

@@ -133,6 +133,35 @@ describe("fake", function() {
         });
     });
 
+    describe(".firstArg", function() {
+        it("should be the first argument from the last call", function() {
+            var f = fake();
+            f(41, 42, 43);
+            assert.equals(f.firstArg, 41);
+
+            f(44, 45);
+            assert.equals(f.firstArg, 44);
+
+            f(46);
+            assert.equals(f.firstArg, 46);
+
+            f(false, true, 47, "string");
+            assert.equals(f.firstArg, false);
+
+            f("string", false, true, 47);
+            assert.equals(f.firstArg, "string");
+
+            f(47, "string", false, true);
+            assert.equals(f.firstArg, 47);
+
+            f(true, 47, "string", false);
+            assert.equals(f.firstArg, true);
+
+            f();
+            assert.isUndefined(f.firstArg);
+        });
+    });
+
     describe(".lastArg", function() {
         it("should be the last argument from the last call", function() {
             var f = fake();