Pass req to getProxyForUrl callback (#353)

This allows more flexibility in the callback, e.g. for inspecting
request headers before deciding on a given proxy.

---------

Co-authored-by: Anna Henningsen <anna@addaleax.net>

.changeset/sixty-carpets-cross.md

@@ -0,0 +1,5 @@
+---
+'proxy-agent': minor
+---
+
+Include ClientRequest in getProxyForUrl parameters for additional flexibility

packages/proxy-agent/src/index.ts

@@ -20,7 +20,10 @@ type ValidProtocol =
 
 type AgentConstructor = new (proxy: string, proxyAgentOptions?: ProxyAgentOptions) => Agent;
 
-type GetProxyForUrlCallback = (url: string) => string | Promise<string>;
+type GetProxyForUrlCallback = (
+	url: string,
+	req: http.ClientRequest
+) => string | Promise<string>;
 
 /**
  * Shorthands for built-in supported types.
@@ -128,7 +131,7 @@ export class ProxyAgent extends Agent {
 			: 'http:';
 		const host = req.getHeader('host');
 		const url = new URL(req.path, `${protocol}//${host}`).href;
-		const proxy = await this.getProxyForUrl(url);
+		const proxy = await this.getProxyForUrl(url, req);
 
 		if (!proxy) {
 			debug('Proxy not enabled for URL: %o', url);

packages/proxy-agent/test/test.ts

@@ -266,15 +266,17 @@ describe('ProxyAgent', () => {
 		it('should call provided function with getProxyForUrl option', async () => {
 			let gotCall = false;
 			let urlParameter = '';
+			let reqParameter: http.ClientRequest | undefined;
 			httpsServer.once('request', function (req, res) {
 				res.end(JSON.stringify(req.headers));
 			});
 
 			const agent = new ProxyAgent({
 				rejectUnauthorized: false,
-				getProxyForUrl: (u) => {
+				getProxyForUrl: (u, r) => {
 					gotCall = true;
 					urlParameter = u;
+					reqParameter = r;
 					return httpsProxyServerUrl.href;
 				},
 			});
@@ -287,6 +289,7 @@ describe('ProxyAgent', () => {
 			assert(httpsServerUrl.host === body.host);
 			assert(gotCall);
 			assert(requestUrl.href === urlParameter);
+			assert(reqParameter?.constructor.name === 'ClientRequest');
 		});
 
 		it('should call provided function with asynchronous getProxyForUrl option', async () => {
@@ -298,7 +301,7 @@ describe('ProxyAgent', () => {
 
 			const agent = new ProxyAgent({
 				rejectUnauthorized: false,
-				getProxyForUrl: async(u) => {
+				getProxyForUrl: async (u) => {
 					gotCall = true;
 					urlParameter = u;
 					await promisify(setTimeout)(1);