Mocking a module

vi.mock()'s path argument needs to resolve to the same file that the module under test is using. If users.js imports <root>/src/client.js, vi.mock()'s path argument needs to match:

// users.js
import client from './client' // => resolves to path/to/client.js

// users.spec.js
vi.mock('../../client.js')    // => resolves to path/to/client.js

It often helps to use path aliases here.

Spying/mocking a function

To spy on or mock a function of the mocked module, do the following in test():

1. Dynamically import the module, which gets the mocked module.
2. Mock the function off of the mocked module reference, optionally returning a mock value. Since client.get() returns axios.get(), which returns a Promise, it makes sense to use mockResolvedValue() to mock the returned data.

// users.spec.js
import { describe, test, expect, vi } from 'vitest'
import UsersAPI from '@/users.js'

vi.mock('@/client')

describe('Users API', () => {
  test('Users API.getUsers', async () => {
    1️⃣
    const client = await import('@/client')

    2️⃣
    const response = { data: [{ id: 1, name: 'john doe' }] }
    client.default.get = vi.fn().mockResolvedValue(response)
    
    const users = await UsersAPI.getUsers()
    expect(client.default.get).toHaveBeenCalled()
    expect(users).toEqual(response)
  })
})

demo