Chapter 4 of 7

Network Mocking & API Testing

Control the network layer: mock unstable third parties, stub edge cases, and even run pure-API tests using Playwright's request context.

13 min guide4 reference questions folded into the guide material

Code samples in

Guided briefing

Network Mocking & API Testing video demo

A practical screen-share style walkthrough for chapter 4, showing how the Playwright idea works in TypeScript and Java.

Briefing focus

Why network mocking matters

This is a structured lesson briefing. Real video/audio can be added later as a media source.

Estimated time

10 min

1Why network mocking matters
2Pure API tests with the request context

Transcript brief

Control the network layer: mock unstable third parties, stub edge cases, and even run pure-API tests using Playwright's request context. The video brief explains the mental model first, then demonstrates the workflow using the course code samples, and finishes with reliability checks you can apply in CI.

Key takeaways

Translate the concept into a maintainable Playwright test.
Understand the TypeScript and Java equivalents without changing the test intent.
Avoid the common source of flaky or slow end-to-end tests for this topic.

Why network mocking matters

Modern apps depend on dozens of external APIs. One flaky third-party is enough to break your suite. Playwright's route() API lets you intercept, modify or stub network calls — perfect for edge cases that are hard to reproduce live (500s, slow responses, deprecated schemas).

test('checkout handles Stripe 500 gracefully', async ({ page }) => {
  await page.route('**/v1/payment_intents', (route) =>
    route.fulfill({
      status: 500,
      contentType: 'application/json',
      body: JSON.stringify({ error: 'internal_error' }),
    })
  );

  await page.goto('/checkout');
  await page.getByRole('button', { name: 'Pay now' }).click();
  await expect(page.getByRole('alert')).toContainText(/payment failed/i);
});

Stub a failing third-party call to test error states.

Pure API tests with the request context

Playwright can drive APIs directly — no browser needed. Use it to seed data, sanity-check contracts, or run an API-only pyramid layer alongside UI tests.

import { test, expect } from '@playwright/test';

test('GET /api/courses returns ISTQB', async ({ request }) => {
  const res = await request.get('/api/courses?slug=istqb-foundation-essentials');
  expect(res.status()).toBe(200);
  const json = await res.json();
  expect(json.course.title).toContain('ISTQB');
});

An API-only test.

Real-life scenario · E-commerce

Testing a Stripe outage without taking production down

Situation. A retailer's checkout went down for 9 minutes during Black Friday because their app crashed when Stripe returned 503. Their new Playwright test stubs a 503 from `**/v1/payment_intents` and asserts a graceful fallback message is shown. The test now runs on every PR.

Lesson. Mock your dependencies at the network boundary. You gain test speed, determinism and the ability to verify edge-cases you can't provoke safely in production.

Use HAR files for complex mocks

page.routeFromHAR('fixture.har') replays a previously-recorded network session. Ideal when you need to mock 40 endpoints with realistic payloads.