feat: support optional preload attributes

Support adding additional attributes to preload Link headers, to signal
to downstream servers if the resources should be converted to HTTP/2
Server Push.

The Preload specification defines "nopush" to signal that the downstream
server should not push the resources.[0] Fastly also supports
"x-http2-push-only"[1], other downstreams may implement other
attributes.

[0]: https://www.w3.org/TR/preload/#server-push-http-2
[1]: https://docs.fastly.com/guides/performance-tuning/http2-server-push

[[email protected]: added commit message]
Signed-off-by: [email protected]

README.md

@@ -26,6 +26,10 @@ express()
 
   Object passed straight to [`lru-cache`][lru-cache]. It is highly recommended to set `cache.max` to an integer.
 
+* **attributes**: `Array<String>`
+
+  List of custom attributes that should be added to the Preload Link headers.
+
 ## License
 
 [![MIT License](https://img.shields.io/badge/license-MIT-blue.svg?style=flat-square)](https://www.tldrlegal.com/l/mit) see `LICENSE.md`.

index.js

@@ -20,9 +20,11 @@ module.exports = function netjet(options) {
     scripts: true,
     styles: true,
     cache: {},
+    attributes: [],
   });
 
   var cache = new LRU(options.cache);
+  var attributes = [''].concat(options.attributes).join('; ');
 
   return function netjetMiddleware(req, res, next) {
     function appendHeader(field, value) {
@@ -60,7 +62,8 @@ module.exports = function netjet(options) {
               (addBaseHref ? baseTag[0] : '') +
               encodeRFC5987(unescape(url)) +
               '>; rel=preload; as=' +
-              asType
+              asType +
+              attributes
           );
         });
     }

test/preload.js

@@ -321,6 +321,35 @@ describe('preload', function() {
         });
     });
   });
+
+  describe('attributes', function() {
+    it('should add a custom attribute', function(done) {
+      var server = createServer({
+        attributes: ['nopush'],
+      });
+
+      request(server)
+        .get('/404')
+        .expect('Content-Type', 'text/html; charset=utf-8')
+        .expect('Link', '</images/droids.png>; rel=preload; as=image; nopush')
+        .expect(404, done);
+    });
+
+    it('should add the custom attributes', function(done) {
+      var server = createServer({
+        attributes: ['nopush', 'x-http2-push-only'],
+      });
+
+      request(server)
+        .get('/404')
+        .expect('Content-Type', 'text/html; charset=utf-8')
+        .expect(
+          'Link',
+          '</images/droids.png>; rel=preload; as=image; nopush; x-http2-push-only'
+        )
+        .expect(404, done);
+    });
+  });
 });
 
 function createServer(options) {