How to spy on a default exported function with Jest?

ReactjsUnit TestingMockingJestjsSpy

Reactjs Problem Overview

Suppose I have a simple file exporting a default function:

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

export default uniqueIdGenerator;

Which I would use like this:

import uniqueIdGenerator from './UniqueIdGenerator';
// ...

I want to assert in my test that this method was called while keeping the original functionality. I'd do that with jest.spyOn however, it requires an object as well as a function name as parameters. How can you do this in a clean way? There's a similar GitHub issue for jasmine for anyone interested.

Reactjs Solutions

Solution 1 - Reactjs

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');

Solution 2 - Reactjs

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(() => {

Solution 3 - Reactjs

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')

Solution 4 - Reactjs

Mock only the default export, or any other export, but keep remaining exports in module as original:

import myDefault, { myFunc, notMocked } from "./myModule";

jest.mock("./myModule", () => {
  const original = jest.requireActual("./myModule");
  return {
    __esModule: true,
    default: jest.fn(),
    myFunc: jest.fn()

describe('my description', () => {
  it('my test', () => {
    myDefault.mockImplementation(() => 5);
    expect(notMocked()).toBe("i'm not mocked!");

Solution 5 - Reactjs

Here is a way of doing it for a default export without modifying the import (or even needing an import in the test at all):

const actual = jest.requireActual("./UniqueIdGenerator");
const spy = jest.spyOn(actual, "default");

Solution 6 - Reactjs

What worked for me was a combination of the answer from Janne Annala and OP's own solution. All I wanted to test was that the helper method was called with the correct parameters as I had already written a test for the helper method and it didn't have any bearing on my subsequent test:

// myHelperMethod.js

export const myHelperMethod = (param1, param2) => { // do something with the params };
// someOtherFileUsingMyHelperMethod.js

import * as MyHelperMethod from '../myHelperMethod';

jest.mock('../myHelperMethod', () => ({
  myHelperMethod: jest.fn(),

let myHelperMethodSpy = jest.spyOn(MyHelperMethod, 'myHelperMethod');

// ...
// some setup
// ...

test(() => {
  expect(myHelperMethodSpy).toHaveBeenCalledWith(param1, param2);


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Content TypeOriginal AuthorOriginal Content on Stackoverflow
QuestionthisismydesignView Question on Stackoverflow
Solution 1 - ReactjsthisismydesignView Answer on Stackoverflow
Solution 2 - Reactjsalex.spriView Answer on Stackoverflow
Solution 3 - ReactjsJanne AnnalaView Answer on Stackoverflow
Solution 4 - ReactjsdeckeleView Answer on Stackoverflow
Solution 5 - ReactjsHubrisView Answer on Stackoverflow
Solution 6 - ReactjsAdamJBView Answer on Stackoverflow