fix(res.send): send a raw ArrayBuffer as binary instead of an empty JSON object

lib/response.js

@@ -119,7 +119,7 @@ res.links = function(links) {
  *     res.send({ some: 'json' });
  *     res.send('<p>some html</p>');
  *
- * @param {string|number|boolean|object|Buffer} body
+ * @param {string|number|boolean|object|Buffer|ArrayBuffer} body
  * @public
  */
 
@@ -148,7 +148,11 @@ res.send = function send(body) {
     case 'object':
       if (chunk === null) {
         chunk = '';
-      } else if (ArrayBuffer.isView(chunk)) {
+      } else if (ArrayBuffer.isView(chunk) || isArrayBuffer(chunk)) {
+        // `ArrayBuffer.isView()` is false for a raw ArrayBuffer, so its bytes
+        // are copied into a Buffer; otherwise it would fall through to
+        // `res.json()` below and be serialized as an empty object.
+        chunk = ArrayBuffer.isView(chunk) ? chunk : Buffer.from(chunk);
         if (!this.get('Content-Type')) {
           this.type('bin');
         }
@@ -1011,6 +1015,23 @@ function sendfile(res, file, options, callback) {
   file.pipe(res);
 }
 
+/**
+ * Determine if `obj` is a raw ArrayBuffer or SharedArrayBuffer.
+ *
+ * `ArrayBuffer.isView()` only detects views over a buffer (TypedArray and
+ * DataView), and `instanceof` is not reliable across realms, so the internal
+ * class of the object is checked instead.
+ *
+ * @param {*} obj
+ * @return {boolean}
+ * @private
+ */
+
+function isArrayBuffer (obj) {
+  var tag = Object.prototype.toString.call(obj);
+  return tag === '[object ArrayBuffer]' || tag === '[object SharedArrayBuffer]';
+}
+
 /**
  * Stringify JSON, like JSON.stringify, but v8 optimized, with the
  * ability to escape characters that can trigger HTML sniffing.

test/res.send.js

@@ -231,6 +231,32 @@ describe('res', function(){
     })
   })
 
+  describe('.send(ArrayBuffer)', function(){
+    it('should send as application/octet-stream', function(done){
+      var app = express();
+      app.use(function(req, res){
+        res.send(new TextEncoder().encode('hey').buffer);
+      });
+      request(app)
+      .get('/')
+      .expect(200)
+      .expect('Content-Type', 'application/octet-stream')
+      .expect(utils.shouldHaveBody(Buffer.from('hey')))
+      .end(done)
+    })
+
+    it('should not override an existing Content-Type', function(done){
+      var app = express();
+      app.use(function(req, res){
+        res.set('Content-Type', 'text/plain').send(new TextEncoder().encode('hey').buffer);
+      });
+      request(app)
+      .get('/')
+      .expect('Content-Type', 'text/plain; charset=utf-8')
+      .expect(200, 'hey', done);
+    })
+  })
+
   describe('.send(Object)', function(){
     it('should send as application/json', function(done){
       var app = express();