Add .lastArg and .callback to fake and call

lib/sinon/fake.js

@@ -32,8 +32,25 @@ function cleanProxy(f) {
     return f;
 }
 
+var uuid = 0;
 function wrapFunc(f) {
-    return cleanProxy(spy(f));
+    var fakeInstance = function () {
+        var lastArg = arguments.length > 0 && arguments[arguments.length - 1] || undefined;
+        var callback = lastArg && typeof lastArg === "function" ? lastArg : undefined;
+
+        /* eslint-disable no-use-before-define */
+        p.lastArg = lastArg;
+        p.callback = callback;
+        /* eslint-enable no-use-before-define */
+
+        return f && f.apply(f, arguments);
+    };
+    var p = cleanProxy(spy(fakeInstance));
+
+    p.displayName = "fake";
+    p.id = "fake#" + uuid++;
+
+    return p;
 }
 
 function fake(f) {

test/fake-test.js

@@ -4,6 +4,7 @@ var sinon = require("../lib/sinon.js");
 var fake = sinon.fake;
 var referee = require("@sinonjs/referee");
 var assert = referee.assert;
+var refute = referee.refute;
 
 referee.add("isProxy", {
     assert: function assertIsProxy(actual) {
@@ -50,6 +51,66 @@ describe("fake", function () {
         verifyProxy(fake);
     });
 
+    describe(".callback", function () {
+        it("it should be a reference for the callback in the last call", function () {
+            var f = fake();
+            var callback1 = function () {};
+            var callback2 = function () {};
+
+            f(1, 2, 3, callback1);
+            assert.equals(f.callback, callback1);
+
+            f(1, 2, 3, callback2);
+            assert.equals(f.callback, callback2);
+
+            f(1, 2, 3);
+            assert.equals(f.callback, undefined);
+        });
+    });
+
+    describe(".displayName", function () {
+        it("should be 'fake'", function () {
+            var fakes = [
+                fake(),
+                fake.returns(42),
+                fake.throws(new Error()),
+                fake.resolves(42),
+                fake.rejects(new Error()),
+                fake.yields(42),
+                fake.yieldsAsync(42)
+            ];
+
+            fakes.forEach(function (f) {
+                assert.equals(f.displayName, "fake");
+            });
+        });
+    });
+
+    describe(".id", function () {
+        it("should start with 'fake#'", function () {
+            for (var i = 0; i < 100; i++) {
+                assert.isTrue(fake().id.indexOf("fake#") === 0);
+            }
+        });
+    });
+
+    describe(".lastArg", function () {
+        it("should be the last argument from the last call", function () {
+            var f = fake();
+            f(41, 42, 43);
+            assert.equals(f.lastArg, 43);
+
+            f(44, 45);
+            assert.equals(f.lastArg, 45);
+
+            f(46);
+            assert.equals(f.lastArg, 46);
+
+            f();
+            refute.defined(f.lastArg);
+        });
+    });
+
     describe(".returns", function () {
         it("should return a function that returns the argument", function () {
             var expected = 42;