fix: don't override --port=0 with the default port

lib/chromedriver.js

@@ -40,7 +40,7 @@ function start(args, returnPromise) {
     command = process.platform === "win32" ? "chromedriver.exe" : "chromedriver";
   }
   let port = getPortFromArgs(args);
-  if (!port) {
+  if (port === undefined) {
     args.push("--port=9515");
     port = 9515;
   }
@@ -49,6 +49,9 @@ function start(args, returnPromise) {
   cp.stderr.pipe(process.stderr);
   defaultInstance = cp;
   if (!returnPromise) return cp;
+  // with --port=0 the OS assigns a random port we cannot know here, so there is
+  // nothing to poll; resolve as soon as the process is spawned.
+  if (port === 0) return Promise.resolve(cp);
   const pollInterval = 100;
   const timeout = 10000;
   return tcpPortUsed.waitUntilUsed(port, pollInterval, timeout).then(function () {

tests/start.test.js

@@ -0,0 +1,47 @@
+const child_process = require("node:child_process");
+
+describe("start", () => {
+  /** @type {import('child_process').ChildProcess} */
+  let fakeCp;
+  let spawnSpy;
+  beforeEach(() => {
+    fakeCp = /** @type {any} */ ({
+      stdout: { pipe: jest.fn() },
+      stderr: { pipe: jest.fn() },
+      kill: jest.fn(),
+    });
+    jest.resetModules();
+    spawnSpy = jest.spyOn(child_process, "spawn").mockClear().mockReturnValue(fakeCp);
+  });
+  afterAll(() => jest.restoreAllMocks());
+
+  it("uses the default port when none is passed", () => {
+    const { start } = require("../lib/chromedriver");
+    const args = [];
+    start(args);
+    expect(args).toContain("--port=9515");
+    expect(spawnSpy.mock.calls[0][1]).toContain("--port=9515");
+  });
+
+  it("keeps --port=0 instead of overriding it with the default", () => {
+    const { start } = require("../lib/chromedriver");
+    const args = ["--port=0"];
+    start(args);
+    expect(args).not.toContain("--port=9515");
+    expect(spawnSpy.mock.calls[0][1]).toEqual(["--port=0"]);
+  });
+
+  it("resolves immediately for --port=0 when a promise is requested", async () => {
+    const { start } = require("../lib/chromedriver");
+    const cp = await start(["--port=0"], true);
+    expect(cp).toBe(fakeCp);
+  });
+
+  it("keeps an explicit port", () => {
+    const { start } = require("../lib/chromedriver");
+    const args = ["--port=9222"];
+    start(args);
+    expect(args).not.toContain("--port=9515");
+    expect(spawnSpy.mock.calls[0][1]).toEqual(["--port=9222"]);
+  });
+});