[proxy-agent] Allow asynchronous getProxyForUrl (#275)

This allows users to use an asynchronous getProxyForUrl in case they need
to do asynchronous operations to resolve the correct proxy URL.  This can
be the case, for example, if using Electron.Session.resolveProxy(url).

.changeset/nice-pillows-type.md

@@ -0,0 +1,5 @@
+---
+'proxy-agent': minor
+---
+
+Allow `getProxyForUrl()` option to return a Promise

packages/proxy-agent/src/index.ts

@@ -22,7 +22,7 @@ type ValidProtocol = (typeof PROTOCOLS)[number];
 
 type AgentConstructor = new (...args: never[]) => Agent;
 
-type GetProxyForUrlCallback = (url: string) => string;
+type GetProxyForUrlCallback = (url: string) => string | Promise<string>;
 
 /**
  * Supported proxy types.
@@ -115,7 +115,7 @@ export class ProxyAgent extends Agent {
 			: 'http:';
 		const host = req.getHeader('host');
 		const url = new URL(req.path, `${protocol}//${host}`).href;
-		const proxy = this.getProxyForUrl(url);
+		const proxy = await this.getProxyForUrl(url);
 
 		if (!proxy) {
 			debug('Proxy not enabled for URL: %o', url);

packages/proxy-agent/test/test.ts

@@ -2,6 +2,7 @@ import * as fs from 'fs';
 import * as http from 'http';
 import * as https from 'https';
 import { URL } from 'url';
+import { promisify } from 'util';
 import { once } from 'events';
 import assert from 'assert';
 import WebSocket, { WebSocketServer } from 'ws';
@@ -287,6 +288,33 @@ describe('ProxyAgent', () => {
 			assert(gotCall);
 			assert(requestUrl.href === urlParameter);
 		});
+
+		it('should call provided function with asynchronous getProxyForUrl option', async () => {
+			let gotCall = false;
+			let urlParameter = '';
+			httpsServer.once('request', function (req, res) {
+				res.end(JSON.stringify(req.headers));
+			});
+
+			const agent = new ProxyAgent({
+				rejectUnauthorized: false,
+				getProxyForUrl: async(u) => {
+					gotCall = true;
+					urlParameter = u;
+					await promisify(setTimeout)(1);
+					return httpsProxyServerUrl.href;
+				},
+			});
+			const requestUrl = new URL('/test', httpsServerUrl);
+			const res = await req(requestUrl, {
+				agent,
+				rejectUnauthorized: false,
+			});
+			const body = await json(res);
+			assert(httpsServerUrl.host === body.host);
+			assert(gotCall);
+			assert(requestUrl.href === urlParameter);
+		});
 	});
 
 	describe('"ws" module', () => {