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, 8);

export default uniqueIdGenerator;

Which I would use like this:

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

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');