How to mock default package in dependency with Jest? - jestjs

Question

My dependency includes os and fs with require. I didn't notice any issues with my unit tests until I ran in a Jenkins env which gave this error:
StorageDirectoryResolutionError: Storage directory resolution failedUnsupportedFilePlatform: Platform not supported for file operations
I updated my Jest config to be like this:
moduleNameMapper: {
'^os$': '<rootDir>/tests/dependency-mocks/os.js',
'^fs$': '<rootDir>/tests/dependency-mocks/fs.js',
},
I am mocking os and fs, even though error is around fs, to ensure things are same between my machine and Jenkins.
My os.js is tiny:
module.exports = {
homedir: () => '/user/home',
platform: () => 'darwin',
release: () => 'some release',
};
I have an os.ts file in __mocks__ like this:
export default {
homedir: () => '/user/home',
platform: jest.fn().mockReturnValue('darwin'),
release: jest.fn().mockReturnValue('some release'),
};
My actual file has this issue:
const os = require('os');
console.log(os); // my exported object is on a default key
{ default:
{ homedir: [Function: homedir],
platform:
{ [Function: mockConstructor]
_isMockFunction: true,
getMockImplementation: [Function],
mock: [Getter/Setter],
mockClear: [Function],
mockReset: [Function],
mockRestore: [Function],
mockReturnValueOnce: [Function],
mockResolvedValueOnce: [Function],
mockRejectedValueOnce: [Function],
mockReturnValue: [Function],
mockResolvedValue: [Function],
mockRejectedValue: [Function],
mockImplementationOnce: [Function],
mockImplementation: [Function],
mockReturnThis: [Function],
mockName: [Function],
getMockName: [Function] },
release:
{ [Function: mockConstructor]
_isMockFunction: true,
getMockImplementation: [Function],
mock: [Getter/Setter],
mockClear: [Function],
mockReset: [Function],
mockRestore: [Function],
mockReturnValueOnce: [Function],
mockResolvedValueOnce: [Function],
mockRejectedValueOnce: [Function],
mockReturnValue: [Function],
mockResolvedValue: [Function],
mockRejectedValue: [Function],
mockImplementationOnce: [Function],
mockImplementation: [Function],
mockReturnThis: [Function],
mockName: [Function],
getMockName: [Function] } } }
This is where it gets very weird. If I do NOT use moduleNameMapper, the dependency does NOT use my mock file. When I do use moduleNameMapper it ignores the .js file for moduleNameMapper and goes to my .ts file in __mocks__. I realized this when I added a random key to the mock and saw the update in console log.
I need the dependency to use os.release() without the object being nested within default.

Answer 1

The answer was really simple. Changed export default {} in mock files to module.exports = {}.
My project uses import/export for nearly everything. Due to the system it runs on, we have to use const fs = require('fs') instead of import fs from 'fs'; The mocks using export had no issue until moduleNameMapper got thrown into the mix.