Is there a recommended way to mock fetch?

[dusty-phillips]

I've got a (bun + hono) api that interacts with several different REST APIs. Some of those APIs are easy to test; I control the code that calls fetch and can mock it sanely. But others are using SDKs that call fetch on my behalf (sometimes the global fetch, other times node-fetch).

Is there a good way to reliably mock fetch in these third-party projects? I can mock the individual SDK callsites if I have to, but it would be easier to mock fetch. I've tried several incantations of mock.module and even setting global.fetch = mock(), but haven't had much luck.

[dusty-phillips]

I wouldn't call it "recommended" but this seems to work:

import { afterEach, mock } from "bun:test"

export const mockFetch = mock()

global.fetch = mockFetch
mock.module("node-fetch", () => mockFetch)

afterEach(() => {
  mockFetch.mockReset()
})

Then in a test file:

import { mockFetch } from "./mock-fetch-preload"

test("Fetch gets mocked again", async () => {
  mockFetch.mockReturnValueOnce(Promise.resolve(new Response("hello world")))
  const response = await <some function that calls fetch>
  expect(mockFetch).toHaveBeenCalledTimes(1)
  expect(mockFetch).toHaveBeenCalledWith(...)
  //etc
})

It needs to be run with bun test --preload ./path/to/module.ts and seems to be mocking reliably... so far.

[vitch]

This gives hope there is something better but I guess not built in...

I did just come across bun-bagel which sounds promising and I'm going to try out.

[nicholasoxford]

I would recommend bun-bagel to anyone coming across this thread. Within minutes I had fetch mocked the way I imagined

This gives hope there is something better but I guess not built in...

I did just come across bun-bagel which sounds promising and I'm going to try out.