Playwright+Nock: Possible to start server within the same process as nock?

[niklasHagner]

I'm writing my first Playwright test for my serverside-rendering node application.

Struggling with how to start my node application in a way that makes it run within the same process as the nock.

The most common Playwright tests seem to use page.goto("/") to perform a request, and this assumes that a server has already been started.

Here's an example Playwright-test that starts the node server via http.createServer(app):

const { test, expect } = require("@playwright/test");
const http = require("http");
const app = setupMyNodeExpressApp(); //basically does `return express()`
test("My test", async ({ page, context }) => {
  nock("https://my-api.com").get("/").reply(200, { fakeData:true }; //nock an api which is called serverside on all routes of my app
  http.createServer(app).listen(); //start node app
  await page.goto("/"); //make a request via playwright
  await expect(page).toHaveScreenshot("myTest.png", { fullPage: true }); //take screenshot via playwright
}

In this test the nock doesn't apply to requests from the node app. I guess this is because http.createServer(app) starts the node app as a separate process, while nock is running on another process.

I've looked into alternative ways of starting node apps via Playwright, one alternative is to let playwright.config.js start the node process, as described in https://playwright.dev/docs/test-configuration. But that would of course also start the node app as a separate process so it doesn't help

webServer: {
    command: 'npm run start',
    url: 'http://127.0.0.1:3000',
}

---

I am confident that my node application source code is compatible with nock as long as they're running within the same process. Because I've got a large well-established test suite for the same node application where I successfully used nock in combination with supertest

In this existing test-suite I can pass my application to supertest via supertest(app).get("/") and that makes my node app run within the same process as the nock.

Example of supertest code

const supertest = require("supertest");
const app = setupMyNodeExpressApp(); //basically does `return express()`
nock("https://my-api.com").get("/").reply(200, { fakeData:true }; //nock an api which is called serverside on all routes of my app
supertest(app).get("/").expect(200).end((err, response) => { doSomething(); }) //let supertest start the node app and make a request

This works great. Requests from my node server do respond with fakeData accoding to my nock.

---

Wondering if someone knows
a) Different way to start my node app within the same process, rather than http.createServer(app)
b) Different Playwright methods of using a node app within the same process as the test code
c) Different way of using nock
d) Confirm that it's simply a dead end to attempt to use Nock in combination with Playwright for a serverside rendered node app

PS: If I only needed to mock clientside requests (requests from the browser) I know Playwright provides it's own mock methods, https://ray.run/blog/how-to-mock-http-traffic-in-playwright-test. But as far as I can tell these are just for clientside/browser requests so they are useless for mocking internal serverside requests from within my node app.

DS: The main reason I'm writing this question here in github.com/nock is that I've got a ton of nock-code already thanks to my existing test-suite and I'd like to reuse it. While Playwright is new to me.

[gr2m]

It should be possible, I've done it plenty of times. Note that nock currently cannot intercept requests sent using Node 18+'s native fetch method, maybe that's why it's not working for you?

[niklasHagner]

Thanks for your reply.
I understand the concern as I've seen other Github-issues here where native fetch in Node18+ was the problem. I am actually running node 18.16 but not using native fetch, I am using a custom wrapper built on top of https://www.npmjs.com/package/got

I can with confidence say that my node app source code does work in combination with nock as long as the app is running on the same process as nock. I have a large, established test-suite (using mocha, supertest and nock) which has been running on the same node codebase for years, and is trusted as a part of our workflow. I've also double-checked.

The Playwright-test I'm adding now is part of the same codebase so it runs the same node application source code.

[gr2m]

Yes if you use got it should work just fine.

I'm sorry I don't know what the problem might be in your case, I wish I could help you better. I hope you find out what's wrong. When you do, please let us know 🙏🏼

[niklasHagner]

I just got a tip from a colleague that they've managed to use Playwright with serverside mocking.
They are using the Playwright concept of 'fixtures' https://playwright.dev/docs/test-fixtures

This basically allows you to do stuff like

const base = require('@playwright/test');
base.test.extend(something);

where something is an object that can contain properties like port, baseURL, server and some interceptor like a mocking library.

My colleague managed to run their node app and mocking lib within a context that can be exposed to Playwright, and ensures that everything runs within the same process.

My colleague uses https://www.npmjs.com/package/msw as a mocking lib, but hopefully a similar concept should be doable with nock as a mocking lib.

EDIT: or maybe not. Seems like msw has a .listen() function.

Used kinda like this

const { setupServer } = require('msw/node');
const interceptor = setupServer();
interceptor.listen();

And according to old issues like #1911 I see nock will not have a .listen()

I'll post an update later if this turns out to be a dead end or a solution.

[gr2m]

And according to old issues like #1911 I see nock will not have a .listen()

yeah not anytime soon I'm afraid. msf is great, especially for browser mocking. Use whatever tool works for you 👍🏼

[mastermatt]

I've never used Playwright, so I'm making a guess by just looking at the express/nock setup you have in the example.

I don't see anywhere where you're making requests to any host except local. So I wouldn't expect nock("https://my-api.com") to match the request. You may have tuned down your example from real values, but looking at just the code provided, you'd need to use nock("http://127.0.0.1")....

Also don't forget about Nock's debug mode to identity why requests weren't matched.

[niklasHagner]

I've concluded that Playwright+Nock probably isn't possible. Went with Playwright+MSW instead