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How to Intercept Requests in Playwright

When we browse the web, a series of HTTP requests and responses are exchanged between our browser and the pages we are visiting. There are scenarios in which it is useful to monitor or manipulate this traffic.

Request interception enables us to observe which requests and responses are being exchanged as part of our script’s execution. For example, this is how we could print them out when we load our test website:

import { test } from '@playwright/test'

test('intercept requests', async ({ page }) => {
  page.on('request', (request) =>
    console.log('>>', request.method(), request.url())
  )
  page.on('response', (response) =>
    console.log('<<', response.status(), response.url())
  )

  await page.goto('https://danube-web.shop/')
})

Run in Checkly

We might want to intervene and filter the outgoing requests. For example, when scraping web pages, we might want to block unnecessary elements from loading in order to speed up the procedure and lower bandwidth usage.

import { test } from '@playwright/test'

test('intercept requests block', async ({ page }) => {
  await page.route('**/*', (route) => {
    return route.request().resourceType() === 'image'
      ? route.abort()
      : route.continue()
  })

  await page.goto('https://danube-web.shop/')
})

Run in Checkly

As a result, you will see the website logo not being loaded.

Similarly, switching the resourceType to stylesheet would result in the target website loading without any CSS styling.

Isolating one or more software components from their dependencies makes them easier to test. We can do so by substituting interactions with such dependencies with simulated, simplified ones. This is also known as stubbing.

Playwright makes it easy for us, as for every request we can intercept we also can stub a response.

Every time we load it, our test website is sending a request to its backend to fetch a list of best selling books. For our example, we are going to intercept this response and modify it to return a single book we define on the fly.

import { test } from '@playwright/test'

test('intercept response', async ({ page }) => {
  const mockResponseObject = [
    {
      id: 1,
      title: 'How to Mock a Response',
      author: 'A. Friend',
      genre: 'business',
      price: '0.00',
      rating: '★★★★★',
      stock: 65535
    }
  ]

  await page.route('https://danube-web.shop/api/books', (route) =>
    route.fulfill({
      contentType: 'application/json',
      body: JSON.stringify(mockResponseObject)
    })
  )

  await page.goto('https://danube-web.shop/')
  await page.screenshot({ path: 'screenshot.png' })
})

Run in Checkly

Here is what the homepage will look like with our stubbed response:

Run the above examples as follows:

npx playwright test request-interception.ts

1. Playwright gives us control over outgoing HTTP requests.
2. Playwright can easily stub HTTP responses.

1. Official documentation on this topic from Playwright.
2. Mocks Aren’t Stubs by Martin Fowler.