Merge pull request #11 from tschaub/preserve-arity

Only call callback with two args if return is undefined.

lib/binding.js

@@ -27,7 +27,11 @@ function maybeCallback(callback, thisArg, func) {
       err = e;
     }
     process.nextTick(function() {
-      callback(err, val);
+      if (val === undefined) {
+        callback(err);
+      } else {
+        callback(err, val);
+      }
     });
   } else {
     return func.call(thisArg);

test/lib/index.spec.js

@@ -1637,6 +1637,20 @@ describe('Mocking the file system', function() {
       });
     });
 
+    it('calls callback with a single argument on success', function(done) {
+      fs.mkdir('parent/arity', function(err) {
+        assert.equal(arguments.length, 1);
+        done();
+      });
+    });
+
+    it('calls callback with a single argument on failure', function(done) {
+      fs.mkdir('parent', function(err) {
+        assert.instanceOf(err, Error);
+        done();
+      });
+    });
+
   });
 
   describe('fs.mkdirSync(path, [mode])', function() {