Mocking Axios calls using Moxios in order to test API calls

I have the following custom Axios instance:

import axios from 'axios'

export const BASE_URL = 'http://jsonplaceholder.typicode.com'

export default axios.create({
  baseURL: BASE_URL
})

With the corresponding service:

import http from './http'

export async function fetchUserPosts(id) {
  const reponse = await http.get(`/users/${id}/posts`)
  return reponse.data
}

And this is the test for said service:

import moxios from 'moxios'
import sinon from 'sinon'
import http from '@/api/http'
import { fetchUserPosts } from '@/api/usersService'

describe('users service', () => {
  beforeEach(() => {
    moxios.install(http)
  })

  afterEach(() => {
    moxios.uninstall(http)
  })

  it('fetches the posts of a given user', (done) => {
    const id = 1
    const expectedPosts = ['Post1', 'Post2']

    moxios.stubRequest(`/users/${id}/posts`, {
      status: 200,
      response: expectedPosts
    })

    const onFulfilled = sinon.spy()
    fetchUserPosts(1).then(onFulfilled)

    moxios.wait(() => {
      expect(onFulfilled.getCall(0).args[0].data).toBe(expectedPosts)
      done()
    })
  })
})

Which when executed using Karma + Jasmine raises the following error:

Uncaught TypeError: Cannot read property 'args' of null thrown

What I would like to test is that when the endpoint /users/{id}/posts is hit a mocked response is sent back. All this while using my custom axios instance http.

I've tried stubbing as the first example of the documentation of moxios shows. However I don't think that fits my use case, as I would like to check that the request is formed correctly in my service.

I've also tried with the following code, which works as expected, however I would like to test my service (which the following code does not do):

import axios from 'axios'
import moxios from 'moxios'
import sinon from 'sinon'

describe('users service', () => {
  beforeEach(() => {
    moxios.install()
  })

  afterEach(() => {
    moxios.uninstall()
  })

  it('fetches the posts of a given user', (done) => {
    const id = 1
    const expectedPosts = ['Post1', 'Post2']

    moxios.stubRequest(`/users/${id}/posts`, {
      status: 200,
      response: expectedPosts
    })

    const onFulfilled = sinon.spy()
    axios.get(`/users/${id}/posts`).then(onFulfilled)

    moxios.wait(() => {
      expect(onFulfilled.getCall(0).args[0].data).toBe(expectedPosts)
      done()
    })
  })
})

Any ideas on how could I fix the error?

I know that is an old question but as I came across with it with the same problem, I'll post my approach:

You don't need to use sinon in this case, as you have an instance of axios, use it to configure moxios (as you already have done)

beforeEach(() => {
    moxios.install(http)
  })
  afterEach(() => {
    moxios.uninstall(http)
})

then you test your method like this:

it('test get', async () => {
     const expectedPosts = ['Post1', 'Post2']

     moxios.wait(() => {
          const request = moxios.requests.mostRecent()
          request.respondWith({ status: 200, response: expectedPosts }) //mocked response
     })

     const result = await fetchUserPosts(1)
     console.log(result) // ['Post1','Post2'] 
     expect(result).toEqual(expectedPosts)
})

That's it.

Regards