Mocking a module

Jest can intercept require and import before they are resolved and change the modules with a custom implementation. This allows us to test scenarios that would otherwhise be impossible to test or very difficult

// users.js
const axios = require('axios')

module.exports = function getUsers() {
  return axios.get('/users.json').then(resp => resp.data)
}

// users.test.js
import axios from 'axios'
import Users from './users'

jest.mock('axios')

test('should fetch users', () => {
  const users = [{ name: 'Bob' }]
  const resp = { data: users }
  axios.get.mockResolvedValue(resp)

  // or you could use the following depending on your use case:
  // axios.get.mockImplementation(() => Promise.resolve(resp))

  return getUsers().then(data => expect(data).toEqual(users))
})

in the code above, thanks to jest.mock('axios') axios is not the real axios module, but a mock function that allow us to mock the return value.

Author: Jaga Santagostino