FetchEvent#waitUntil in Jest context

[jamesarosen]

Problem

The promises passed to FetchEvent.prototype.waitUntil cannot be retrieved via Response.prototype.waitUntil(). There doesn't seem to be any other way to access them either, but if there is, the Writing and Running Tests docs don't mention it.

Configuration

// jest.config.js
module.exports = {
  testEnvironment: 'miniflare',
}

Example Code

describe('waitUntil', () => {
  it('caches things', async () => {
    async function myHandler(fetchEvent) {
      const { url } = fetchEvent.request
      fetchEvent.waitUntil(caches.default.put(url, new Response('written to cache')))
      return new Response('returned from worker')
    }

    // build a FetchEvent:
    const url = 'https://example.com/cache-something'
    const request = new Request(url)
    const fetchEvent = new FetchEvent('fetch', { request })

    // get a response for it:
    const response = await myHandler(fetchEvent)

    // the response should have the Promises from `fetchEvent.waitUntil`:
    const awaited = await response.waitUntil()
    expect(awaited[0]).toBeTruthy() // this fails; awaited is []
    expect(await awaited[0].text()).toBe('written to cache')

    // and the cache-write should have completed:
    const cachedResponse = await caches.default.match(url)
    expect(cachedResponse).toBeTruthy()
    expect(await cachedResponse.text()).toBe('written to cache')
  })
})

[jamesarosen]

The implementation of FetchEvent.prototype.waitUntil stores the promises, but they're locked away behind a Symbol:

miniflare/packages/core/src/standards/event.ts

Lines 80 to 85 in dc55621

	waitUntil(promise: Awaitable<any>): void {
	if (!(this instanceof FetchEvent)) {
	throw new TypeError("Illegal invocation");
	}
	this[kWaitUntil].push(Promise.resolve(promise));
	}

[mrbbot]

Hey! 👋 Thanks for raising this. Will try get a getWaitUntil(event) method added and exposed to tests soon. 🙂

[jamesarosen]

@mrbbot any news on this? Can I help?

[mrbbot]

Hey! 👋 Apologies for the delayed response. I've recently returned from a long holiday and am catching up on issues now.

@jamesarosen I'd be very happy to accept a pull-request for this, otherwise I can do it. You'll need to add a getMiniflareWaitUntil method here...

miniflare/packages/jest-environment-miniflare/src/index.ts

Lines 222 to 235 in 5a48285

	global.getMiniflareBindings = () => bindings;
	global.getMiniflareDurableObjectStorage = async (id: DurableObjectId) => {
	const plugin = (await mf.getPlugins()).DurableObjectsPlugin;
	const storage = mf.getPluginStorage("DurableObjectsPlugin");
	const state = await plugin.getObject(storage, id);
	return state.storage;
	};
	global.flushMiniflareDurableObjectAlarms = async (
	ids?: DurableObjectId[]
	): Promise<void> => {
	const plugin = (await mf.getPlugins()).DurableObjectsPlugin;
	const storage = mf.getPluginStorage("DurableObjectsPlugin");
	return plugin.flushAlarms(storage, ids);
	};

...that accesses the [kWaitUntil] field of FetchEvent | ScheduledEvent. You'll probably want to export a getWaitUntil helper from this file, as kWaitUntil shouldn't be exported.

miniflare/packages/core/src/standards/event.ts

Line 37 in 5a48285

const kWaitUntil = Symbol("kWaitUntil");

[CraigglesO]

Just want to add a note that when using cache.put a max-age must be provided otherwise the cache tosses it out immediately:

function testResponse(body) {
  return new Response(body, { headers: { "Cache-Control": "max-age=3600" } });
}

...

event.waitUntil(caches.default.put(url, testResponse("example cache")));

[jamesarosen]

@CraigglesO I poked at this a bit. I'm glad you did it because your #345 looks much better than what I had!