How to spy on a fakerjs method call

Faker.js allow you to easily create faked data using for example the following:

import * as faker from 'faker'
console.log(faker.lorem.text())

So I tried to mock this library to spy the use of faker.lorem.text():

import * as faker from 'faker'

const mockFakerLoremText = jest.fn()
jest.mock('faker', () => ({
  lorem: {
    text: mockFakerLoremText
  }
}))


it('should have called lorem.text() method', () => {
  faker.lorem.text()

  expect(mockFakerLoremText).toHaveBeenCalledTimes(1)
})

But then I got the following error:

ReferenceError: Cannot access 'mockFakerLoremText' before initialization

So has someone an idea how I can spy on the call of this method .lorem.text()?

Solution

From the docs Calling jest.mock() with the module factory parameter

A limitation with the factory parameter is that, since calls to jest.mock() are hoisted to the top of the file, it's not possible to first define a variable and then use it in the factory

That's why you got the error.

An working example using "jest": "^26.6.3":

index.test.js:

import * as faker from 'faker';

jest.mock('faker', () => ({
  lorem: {
    text: jest.fn(),
  },
}));

it('should have called lorem.text() method', () => {
  faker.lorem.text();

  expect(faker.lorem.text).toHaveBeenCalledTimes(1);
});

unit test result:

 PASS  examples/65924623/index.test.ts
  √ should have called lorem.text() method (3 ms)

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files |       0 |        0 |       0 |       0 |
----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        6.913 s, estimated 7 s