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.

[ImBIOS]

# Unhandled error between tests
-------------------------------
1 | (function (entry, fetcher)
    ^
SyntaxError: Missing 'default' export in module 'node-fetch'.
-------------------------------


 0 pass
 1 fail
 1 error
Ran 1 tests across 1 files. [93.00ms]

[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.

[owenizedd]

The problem with bun-bagel is that it can't mock the return header as you want. it only support body mocking.

[DRFR0ST]

@owenizedd working on it! 😄 DRFR0ST/bun-bagel#7

[rafaell-lycan]

@DRFR0ST is this gonna be a better (simple) version of nock? 💯

I have solved my issues with undici while I couldn't solve them with nock.

Nit tip: To mock response errors isn't working properly, or perhaps the guides are outdated.

@Jarred-Sumner is there an easy way to mock fetch in bun? Perhaps a mock.fetch will be cooked soon?

thx

[DRFR0ST]

@rafaell-lycan initially I've built the library for my personal use, but decided to make it public. Not sure yet where it will go, but I'm semi-actively improving it upon demand, at least as long there is no official solution.

Thanks, I'll look into the response error issue.

Contributions are very welcome and appreciated! 🙏

[kravetsone]

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.

Msw

[wellcaffeinated]

For future googlers. I found bun-bagel to be buggy and feature incomplete. This worked better for me:
https://github.com/aryzing/bun-mock-fetch