Now spies preserve original function arity

spec/core/SpySpec.js

@@ -57,6 +57,24 @@ describe('Spies', function () {
 
       expect(trackSpy.calls.mostRecent().args[0].returnValue).toEqual("return value");
     });
+
+    it("preserves arity of original function", function () {
+      var functions = [
+        function nullary () {},
+        function unary (arg) {},
+        function binary (arg1, arg2) {},
+        function ternary (arg1, arg2, arg3) {},
+        function quaternary (arg1, arg2, arg3, arg4) {},
+        function quinary (arg1, arg2, arg3, arg4, arg5) {},
+        function senary (arg1, arg2, arg3, arg4, arg5, arg6) {}
+      ];
+
+      functions.forEach(function (someFunction, arity) {
+        var spy = jasmineUnderTest.createSpy(someFunction.name, someFunction);
+
+        expect(spy.length).toEqual(arity);
+      });
+    });
   });
 
   describe("createSpyObj", function() {

src/core/Spy.js

@@ -0,0 +1,58 @@
+getJasmineRequireObj().Spy = function (j$) {
+
+  function Spy(name, originalFn) {
+    var args = buildArgs(),
+      /*`eval` is the only option to preserve both this and context:
+        - former is needed to work as expected with methods,
+        - latter is needed to access real spy function and allows to reduce eval'ed code to absolute minimum
+        More explanation here (look at comments): http://www.bennadel.com/blog/1909-javascript-function-constructor-does-not-create-a-closure.htm
+       */
+      /* jshint evil: true */
+      wrapper = eval('(function (' + args + ') { return spy.apply(this, Array.prototype.slice.call(arguments)); })'),
+      spyStrategy = new j$.SpyStrategy({
+        name: name,
+        fn: originalFn,
+        getSpy: function () {
+          return wrapper;
+        }
+      }),
+      callTracker = new j$.CallTracker(),
+      spy = function () {
+        var callData = {
+          object: this,
+          args: Array.prototype.slice.apply(arguments)
+        };
+
+        callTracker.track(callData);
+        var returnValue = spyStrategy.exec.apply(this, arguments);
+        callData.returnValue = returnValue;
+
+        return returnValue;
+      };
+
+    function buildArgs() {
+      var args = [];
+
+      while (originalFn instanceof Function && args.length < originalFn.length) {
+        args.push('arg' + args.length);
+      }
+
+      return args.join(', ');
+    }
+
+    for (var prop in originalFn) {
+      if (prop === 'and' || prop === 'calls') {
+        throw new Error('Jasmine spies would overwrite the \'and\' and \'calls\' properties on the object being spied upon');
+      }
+
+      wrapper[prop] = originalFn[prop];
+    }
+
+    wrapper.and = spyStrategy;
+    wrapper.calls = callTracker;
+
+    return wrapper;
+  }
+
+  return Spy;
+};

src/core/base.js

@@ -62,38 +62,7 @@ getJasmineRequireObj().base = function(j$, jasmineGlobal) {
   };
 
   j$.createSpy = function(name, originalFn) {
-
-    var spyStrategy = new j$.SpyStrategy({
-        name: name,
-        fn: originalFn,
-        getSpy: function() { return spy; }
-      }),
-      callTracker = new j$.CallTracker(),
-      spy = function() {
-        var callData = {
-          object: this,
-          args: Array.prototype.slice.apply(arguments)
-        };
-
-        callTracker.track(callData);
-        var returnValue = spyStrategy.exec.apply(this, arguments);
-        callData.returnValue = returnValue;
-
-        return returnValue;
-      };
-
-    for (var prop in originalFn) {
-      if (prop === 'and' || prop === 'calls') {
-        throw new Error('Jasmine spies would overwrite the \'and\' and \'calls\' properties on the object being spied upon');
-      }
-
-      spy[prop] = originalFn[prop];
-    }
-
-    spy.and = spyStrategy;
-    spy.calls = callTracker;
-
-    return spy;
+    return new j$.Spy(name, originalFn);
   };
 
   j$.isSpy = function(putativeSpy) {

src/core/requireCore.js

@@ -43,6 +43,7 @@ var getJasmineRequireObj = (function (jasmineGlobal) {
     j$.QueueRunner = jRequire.QueueRunner(j$);
     j$.ReportDispatcher = jRequire.ReportDispatcher();
     j$.Spec = jRequire.Spec(j$);
+    j$.Spy = jRequire.Spy(j$);
     j$.SpyRegistry = jRequire.SpyRegistry(j$);
     j$.SpyStrategy = jRequire.SpyStrategy();
     j$.StringMatching = jRequire.StringMatching(j$);