Use WebSocket check in pac-proxy-agent

This mirrors the check in the core `proxy-agent` package and
serves the same purpose.

.changeset/swift-lizards-poke.md

@@ -0,0 +1,5 @@
+---
+'pac-proxy-agent': patch
+---
+
+Properly forward WebSocket requests via PAC agents that resolve to HTTP proxies

packages/pac-proxy-agent/src/index.ts

@@ -180,6 +180,7 @@ export class PacProxyAgent<Uri extends string> extends Agent {
 		opts: AgentConnectOpts
 	): Promise<http.Agent | net.Socket> {
 		const { secureEndpoint } = opts;
+		const isWebSocket = req.getHeader('upgrade') === 'websocket';
 
 		// First, get a generated `FindProxyForURL()` function,
 		// either cached or retrieved from the source
@@ -261,7 +262,7 @@ export class PacProxyAgent<Uri extends string> extends Agent {
 				const proxyURL = `${
 					type === 'HTTPS' ? 'https' : 'http'
 				}://${target}`;
-				if (secureEndpoint) {
+				if (secureEndpoint || isWebSocket) {
 					agent = new HttpsProxyAgent(proxyURL, this.opts);
 				} else {
 					agent = new HttpProxyAgent(proxyURL, this.opts);

packages/pac-proxy-agent/test/test.ts

@@ -174,6 +174,27 @@ describe('PacProxyAgent', () => {
 			assert('via' in data);
 		});
 
+		it('should work over an HTTP proxy for WebSockets', async () => {
+			httpServer.once('request', function (req, res) {
+				res.end(JSON.stringify(req.headers));
+			});
+
+			function FindProxyForURL() {
+				return 'PROXY localhost:PORT;';
+			}
+
+			const uri = `data:,${encodeURIComponent(
+				FindProxyForURL.toString().replace('PORT', proxyServerUrl.port)
+			)}`;
+			const agent = new PacProxyAgent(uri);
+
+			const res = await req(new URL('/test', httpServerUrl), { agent, headers: { upgrade: 'websocket' } });
+			const data = await json(res);
+			assert.equal(httpServerUrl.host, data.host);
+			assert(!('via' in data)); // Used CONNECT rather than plain HTTP proxy
+			assert.equal(data.upgrade, 'websocket');
+		});
+
 		it('should work over a SOCKS proxy', async () => {
 			httpServer.once('request', function (req, res) {
 				res.end(JSON.stringify(req.headers));