add option to specify available encodings

closes expressjs#25

README.md

@@ -46,6 +46,14 @@ the response.
 The default filter function uses the [compressible](https://www.npmjs.com/package/compressible)
 module to determine if `res.getHeader('Content-Type')` is compressible.
 
+##### available
+
+The set of encodings to make available for responses.  This is an array of
+strings passed to the `encoding` function of the [accepts](https://www.npmjs.com/package/accepts)
+module to determine the method of compression used.
+
+The default `available` array is `['gzip', 'deflate', 'identity']`.
+
 ##### threshold
 
 The byte threshold for the response body size before compression is considered

index.js

@@ -39,6 +39,7 @@ function compression(options) {
   var opts = options || {}
 
   var filter = opts.filter || shouldCompress
+  var available = opts.available || ['gzip', 'deflate', 'identity']
   var threshold = typeof opts.threshold === 'string'
     ? bytes(opts.threshold)
     : opts.threshold
@@ -157,7 +158,7 @@ function compression(options) {
 
       // compression method
       var accept = accepts(req)
-      var method = accept.encoding(['gzip', 'deflate', 'identity'])
+      var method = accept.encoding(available)
 
       // negotiation failed
       if (!method || method === 'identity') {

test/compression.js

@@ -544,6 +544,45 @@ describe('compression()', function(){
       .end()
     })
   })
+
+  describe('available', function () {
+    it('should limit the encodings used', function(done){
+      var server = createServer({ available: ['gzip'], threshold: 0 }, function (req, res) {
+        res.setHeader('Content-Type', 'text/plain')
+        res.end('hello, world')
+      })
+
+      request(server)
+      .get('/')
+      .set('Accept-Encoding', 'deflate;q=1.000, gzip;q=0.001')
+      .expect('Content-Encoding', 'gzip', done)
+    })
+
+    it('should prevent compression if no encoding available', function(done){
+      var server = createServer({ available: ['gzip'], threshold: 0 }, function (req, res) {
+        res.setHeader('Content-Type', 'text/plain')
+        res.end('hello, world')
+      })
+
+      request(server)
+      .get('/')
+      .set('Accept-Encoding', 'deflate')
+      .expect(shouldNotHaveHeader('Content-Encoding'))
+      .expect(200, done)
+    })
+
+    it('should not prevent available encodings from being used', function(done){
+      var server = createServer({ available: ['deflate'], threshold: 0 }, function (req, res) {
+        res.setHeader('Content-Type', 'text/plain')
+        res.end('hello, world')
+      })
+
+      request(server)
+      .get('/')
+      .set('Accept-Encoding', 'deflate')
+      .expect('Content-Encoding', 'deflate', done)
+    })
+  })
 })
 
 function createServer(opts, fn) {