How to spy on a default exported function with Jest?

Question

Suppose I have a simple file exporting a default function:

// UniqueIdGenerator.js
const uniqueIdGenerator = () => Math.random().toString(36).substring(2,         


        

Answer 1

I ended up ditching the default export:

// UniqueIdGenerator.js
export const uniqueIdGenerator = () => Math.random().toString(36).substring(2, 8);

And then I could use and spy it like this:

import * as UniqueIdGenerator from './UniqueIdGenerator';
// ...
const spy = jest.spyOn(UniqueIdGenerator, 'uniqueIdGenerator');

Some recommend wrapping them in a const object, and exporting that. I suppose you can also use a class for wrapping.

However, if you can't modify the class there's still a (not-so-nice) solution:

import * as UniqueIdGenerator from './UniqueIdGenerator';
// ...
const spy = jest.spyOn(UniqueIdGenerator, 'default');

Answer 2

In some cases you have to mock the import to be able to spy the default export:

import * as fetch from 'node-fetch'

jest.mock('node-fetch', () => ({
  default: jest.fn(),
}))

jest.spyOn(fetch, 'default')

Answer 3

one could also mock the import and pass the original implementation as mock implementation, like:

import uniqueIdGenerator from './UniqueIdGenerator'; // this import is a mock already

jest.mock('./UniqueIdGenerator.js', () => {
  const original = jest. requireActual('./UniqueIdGenerator')
  return {
     __esModule: true,
     default: jest.fn(original.default)
  }
})

test(() => {
  expect(uniqueIdGenerator).toHaveBeenCalled()
})