Mock default export function with inbuilt node test runner

Question

I'm using the inbuilt node test runner, and I'm unable to mock functions that are exported in a reasonable way.

Let's start with the workaround I've found. If I export a function in this way:

wretchedExportSyntax.js

function wretchedExportSyntax() {
  return false;
}

export default {
  wretchedExportSyntax
}

Then I can write a test that mocks the function fairly easily:

index.test.js

import { test, mock } from 'node:test';
import assert from 'node:assert';

import wretchedExportSyntax from './wretchedExportSyntax.js';

function wrapper() {
  return wretchedExportSyntax.wretchedExportSyntax();
}

test('wretched export syntax', () => {
  mock.method(wretchedExportSyntax, 'wretchedExportSyntax', () => true);
  assert.equal(wretchedExportSyntax.wretchedExportSyntax(), true);
})

test('wrapper function that calls wretched export', () => {
  mock.method(wretchedExportSyntax, 'wretchedExportSyntax', () => true);
  assert.equal(wrapper(), true);
})

However, if my file has only one function, I would much prefer to export it in the following way:

defaultExport.js

export default function () {
  return false;
}

It's not a method, so I can't use mock.method(), and mock.fn() does not seem to work the way I understand it.

import defaultExport from './defaultExport.js'
test.skip('default export syntax', (t) => {
  t.mock.fn(defaultExport, () => true); // Doesn't work
  assert.equal(defaultExport(), true);
})

I've built a reduced test case here, but this issue is relevant both for functions I'm writing, as well as functions I don't have control over, like knex().

How do you mock a function that is default export using the native node test runner?

Answer 1

I referenced this question in the following issue: https://github.com/nodejs/help/issues/4298

The workaround we found since we're using CJS is to leverage this library, waiting for an official way to mock modules: https://github.com/mhweiner/cjs-mock