Don't encode responses with Cache-Control: no-transform

index.js

@@ -29,6 +29,11 @@ var zlib = require('zlib')
 module.exports = compression
 module.exports.filter = shouldCompress
 
+/**
+ * Regex to match no-transform directive in a cache-control header
+ */
+var NO_TRANSFORM_REGEX = /(?:^|,)\s*?no-transform\s*?(?:,|$)/
+
 /**
  * Compress response data with gzip / deflate.
  *
@@ -135,6 +140,18 @@ function compression(options) {
         return
       }
 
+      // Don't compress for Cache-Control: no-transform
+      // https://tools.ietf.org/html/rfc7234#section-5.2.1.6
+      var cacheControl = res.getHeader('Cache-Control')
+      if (cacheControl) {
+        var noTransform = NO_TRANSFORM_REGEX.test(cacheControl)
+        if (noTransform) {
+          nocompress('no-transform')
+          return
+        }
+      }
+
+
       // vary
       vary(res, 'Accept-Encoding')
 

test/compression.js

@@ -20,6 +20,52 @@ describe('compression()', function(){
     .expect(200, done)
   })
 
+  describe("Cache-Control", function() {
+
+    [
+      'no-transform',
+      'public, no-transform',
+      'no-transform, private',
+      'no-transform , max-age=1000',
+      'max-age=1000 , no-transform'
+    ].forEach(function(headerValue) {
+      it('should skip Cache-Control: ' + headerValue, function(done){
+        var server = createServer({ threshold: 0 }, function (req, res) {
+          res.setHeader('Content-Type', 'text/plain')
+          res.setHeader('Cache-Control', headerValue)
+          res.end('hello, world')
+        })
+
+        request(server)
+        .get('/')
+        .set('Accept-Encoding', 'gzip')
+        .expect(shouldNotHaveHeader('Content-Encoding'))
+        .expect(200, done)
+      })
+    });
+
+    [
+      'not-no-transform',
+      'public',
+      'no-transform-thingy'
+    ].forEach(function(headerValue) {
+      it('should not skip Cache-Control: ' + headerValue, function(done){
+        var server = createServer({ threshold: 0 }, function (req, res) {
+          res.setHeader('Content-Type', 'text/plain')
+          res.setHeader('Cache-Control', headerValue)
+          res.end('hello, world')
+        })
+
+        request(server)
+        .get('/')
+        .set('Accept-Encoding', 'gzip')
+        .expect('Content-Encoding', 'gzip')
+        .expect(200, done)
+      })
+    });
+
+  })
+
   it('should skip unknown accept-encoding', function(done){
     var server = createServer({ threshold: 0 }, function (req, res) {
       res.setHeader('Content-Type', 'text/plain')