Fix res.format error when only default provided

fixes #2665

History.md

@@ -1,6 +1,7 @@
 unreleased
 ==========
 
+  * Fix `res.format` error when only `default` provided
   * Fix issue where `next('route')` in `app.param` would incorrectly skip values
   * Use `array-flatten` module for flattening arrays
   * deps: accepts@~1.2.9

lib/response.js

@@ -606,7 +606,9 @@ res.format = function(obj){
   if (fn) delete obj.default;
   var keys = Object.keys(obj);
 
-  var key = req.accepts(keys);
+  var key = keys.length > 0
+    ? req.accepts(keys)
+    : false;
 
   this.vary("Accept");
 

test/res.format.js

@@ -67,6 +67,14 @@ app4.use(function(err, req, res, next){
   res.send(err.status, 'Supports: ' + err.types.join(', '));
 })
 
+var app5 = express();
+
+app5.use(function (req, res, next) {
+  res.format({
+    default: function () { res.send('hey') }
+  });
+});
+
 describe('res', function(){
   describe('.format(obj)', function(){
     describe('with canonicalized mime types', function(){
@@ -102,6 +110,13 @@ describe('res', function(){
         .set('Accept', 'text/html')
         .expect('default', done);
       })
+
+      it('should work when only .default is provided', function (done) {
+        request(app5)
+        .get('/')
+        .set('Accept', '*/*')
+        .expect('hey', done);
+      })
     })
 
     describe('in router', function(){