esmock

esmock provides native ESM import mocking for unit tests. Use examples below as a quick-start guide or use the descriptive and friendly esmock guide here.

esmock must be used with node's experimental --loader

{
  "name": "give-esmock-a-star",
  "type": "module",
  "scripts": {
    "test-uvu": "node --loader=esmock ./node_modules/uvu/bin.js ./spec/",
    "test-ava": "ava --node-arguments=\"--loader=esmock\"",
    "test-mocha": "mocha --loader=esmock --no-warnings"
  }
}

esmock has the following signature

await esmock(
  './to/module.js', // path to target module being tested
  { ...childmocks }, // mock definitions imported by target module
  { ...globalmocks } // mock definitions imported everywhere
);

esmock demonstrated with ava unit test examples

import test from 'ava';
import esmock from 'esmock';

test('should mock local files and packages', async t => {
  const main = await esmock('../src/main.js', {
    stringifierpackage : JSON.stringify,
    '../src/hello.js' : {
      default : () => 'world',
      exportedFunction : () => 'foobar'
    }
  });

  t.is(main(), JSON.stringify({ test : 'world foobar' }));
});

test('should do global instance mocks —third param', async t => {
  const { getFile } = await esmock('../src/main.js', {}, {
    fs : {
      readFileSync : () => 'returns this globally';
    }
  });

  t.is(getFile(), 'returns this globally');
});

test('should mock "await import()" using esmock.p', async t => {
  // using esmock.p, mock definitions are kept in cache
  const doAwaitImport = await esmock.p('../awaitImportLint.js', {
    eslint : { ESLint : cfg => cfg }
  });

  // mock definition is there, in cache, when import is called
  t.is(await doAwaitImport('cfg'), 'cfg');

  esmock.purge(doAwaitImport); // clear cache, if you wish
});

test('should merge "default" value, when safe', async t => {
  const main = await esmock('../src/main.js');

  t.is(main(), main.default()); // use the form you prefer
});

test('should use implicit "default"', async t => {
  const mainA = await esmock('../src/exportsMain.js', {
    '../src/main.js' : () => 'mocked main' // short-hand, smaller
  });
  const mainB = await esmock('../src/exportsMain.js', {
    '../src/main.js' : { default : () => 'mocked main' }
  });

  t.is(mainA(), mainB());
});