SvelteKit Playwright Fetch() Mock

Allows you to mock fetch() requests when running Playwright E2E tests on a SvelteKit codebase.

Playwright already lets you mock browser fetch() requests. This allows you to tell the SvelteKit dev server to deliver mocked responses for fetch() calls it makes while doing Server-Side Rendering.

This can speed up your end-to-end tests and make them more reliable, as well as not require credentials for external APIs to be input to your continuous integration environment.

Installation

npm i sveltekit-playwright-fetch-mock

Usage

src/hooks.server.ts

import type { HandleFetch, Handle } from '@sveltejs/kit'
import { sequence } from '@sveltejs/kit/hooks'
import {
    handle as handleHttpMocks,
    handleFetch as handleMockedResponses,
} from 'sveltekit-playwright-fetch-mock'
import { PUBLIC_LOCAL } from '$env/static/public'

export const handle = sequence(
    handleHttpMocks(PUBLIC_LOCAL === '1'),
) satisfies Handle

export const handleFetch = (async ({ request, fetch, event }) => {
    const mockedResponse = handleMockedResponses(request, event)

    if (mockedResponse) {
        return mockedResponse
    }

    return fetch(request)
}) satisfies HandleFetch

In your local and CI environments, add PUBLIC_LOCAL=1 to your environment.

This is what tells the mock code to be active. You can name this however you like, if you also change the reference in src/hooks.server.ts to match it.

src/app.d.ts

type FetchMocks = Record<
    string,
    {
        status: number
        statusText: string
        headers?: Record<string, string>
        body: string
    }
>

declare namespace App {
    // interface Error {}
    interface Locals {
        fetchMocks: FetchMocks
        isPlaywright: boolean
    }
    // interface PageData {}
    // interface Platform {}
}

And finally in your Playwright test:

import { expect, test } from '@playwright/test'
import { mockFetch } from 'sveltekit-playwright-fetch-mock'

test('Example.com test', async ({ page }) => {
    await mockFetch(page, /^https:\/\/www\.example\.com\//, {
        somedata: 'example',
    })
    await page.goto('/some-local-route-that-fetches-example-dot-com')
    // Your Playwright assertions here.
})

So what this means is that when you go to /some-local-route-that-fetches-example-dot-com in your SvelteKit app, and the +page.server.js or +page.server.ts file for that route calls fetch (using the one that SvelteKit provides you) to get data from https://www.example.com/, this package will intercept that request and return { somedata: "example" }