Introduce the requireProtocolSelection option

doc/ws.md

@@ -328,6 +328,9 @@ This class represents a WebSocket. It extends the `EventEmitter`.
     depending on the `protocolVersion`.
   - `perMessageDeflate` {Boolean|Object} Enable/disable permessage-deflate.
   - `protocolVersion` {Number} Value of the `Sec-WebSocket-Version` header.
+  - `requireProtocolSelection` {Boolean} Specifies whether to treat a missing
+    `Sec-WebSocket-Protocol` header in the server response as an error when
+    subprotocols are requested. Defaults to `true`.
   - `skipUTF8Validation` {Boolean} Specifies whether or not to skip UTF-8
     validation for text and close messages. Defaults to `false`. Set to `true`
     only if the server is trusted.

lib/websocket.js

@@ -650,6 +650,9 @@ module.exports = WebSocket;
  *     permessage-deflate
  * @param {Number} [options.protocolVersion=13] Value of the
  *     `Sec-WebSocket-Version` header
+ * @param {Boolean} [options.requireProtocolSelection=false] Specifies whether
+ *     to treat a missing `Sec-WebSocket-Protocol` header in the server response
+ *     as an error when subprotocols are requested.
  * @param {Boolean} [options.skipUTF8Validation=false] Specifies whether or
  *     not to skip UTF-8 validation for text and close messages
  * @private
@@ -663,6 +666,7 @@ function initAsClient(websocket, address, protocols, options) {
     maxPayload: 100 * 1024 * 1024,
     skipUTF8Validation: false,
     perMessageDeflate: true,
+    requireProtocolSelection: true,
     followRedirects: false,
     maxRedirects: 10,
     ...options,
@@ -959,7 +963,7 @@ function initAsClient(websocket, address, protocols, options) {
       } else if (!protocolSet.has(serverProt)) {
         protError = 'Server sent an invalid subprotocol';
       }
-    } else if (protocolSet.size) {
+    } else if (protocolSet.size && opts.requireProtocolSelection) {
       protError = 'Server sent no subprotocol';
     }
 

test/websocket.test.js

@@ -1402,6 +1402,29 @@ describe('WebSocket', () => {
       });
     });
 
+    it('honors the `requireProtocolSelection` option', (done) => {
+      const wss = new WebSocket.Server({
+        handleProtocols() {},
+        server
+      });
+
+      wss.on('connection', (ws) => {
+        assert.strictEqual(ws.protocol, '');
+        ws.on('close', () => wss.close(done));
+      });
+
+      const ws = new WebSocket(
+        `ws://localhost:${server.address().port}`,
+        'foo',
+        { requireProtocolSelection: false }
+      );
+
+      ws.on('open', () => {
+        assert.strictEqual(ws.protocol, '');
+        ws.close();
+      });
+    });
+
     it('honors the `createConnection` option', (done) => {
       const wss = new WebSocket.Server({ noServer: true, path: '/foo' });