esmock

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esmock provides native ESM import mocking for unit tests. Use examples below as a quick-start guide, see the descriptive and friendly esmock guide here, or browse esmock's test runner examples.

esmock is used with node's --loader

{
  "name": "give-esmock-a-star",
  "type": "module",
  "scripts": {
    "test": "node --loader=esmock --test",
    "test-mocha": "mocha --loader=esmock",
    "test-tap": "NODE_OPTIONS=--loader=esmock tap",
    "test-ava": "NODE_OPTIONS=--loader=esmock ava",
    "test-uvu": "NODE_OPTIONS=--loader=esmock uvu spec",
    "test-tsm": "node --loader=tsm --loader=esmock --test *ts",
    "test-ts-node": "node --loader=ts-node/esm --loader=esmock --test *ts",
    "test-jest": "NODE_OPTIONS=--loader=esmock jest"
  },
  "jest": {
    "runner": "jest-light-runner"
  },
}

esmock has the below signature

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

note: esmock is unable to mock some modules, due to missing support for some package.json export expressions. This area is improving —see the wiki for details.

esmock examples

import test from 'node:test'
import assert from 'node:assert/strict'
import esmock from 'esmock'

test('should mock local files and packages', async () => {
  const main = await esmock('../src/main.js', {
    stringifierpackage: JSON.stringify,
    '../src/hello.js': {
      default: () => 'world',
      exportedFunction: () => 'foo'
    }
  })

  assert.strictEqual(main(), JSON.stringify({ test: 'world foo' }))
})

test('should do global instance mocks —third param', async () => {
  const { getFile } = await esmock('../src/main.js', {}, {
    fs: {
      readFileSync: () => 'returns this globally'
    }
  })

  assert.strictEqual(getFile(), 'returns this globally')
})

test('should mock "await import()" using esmock.p', async () => {
  // using esmock.p, mock definitions are kept in cache
  const doAwaitImport = await esmock.p('../awaitImportLint.js', {
    eslint: { ESLint: cfg => cfg }
  })

  // mock definition is returned from cache, when import is called
  assert.strictEqual(await doAwaitImport('cfg'), 'cfg')
  // a bit more info are found in the descriptive guide
})

// a "partial mock" merges the new and original definitions
test('should suppport partial mocks', async () => {
  const pathWrap = await esmock('../src/pathWrap.js', {
    path: { dirname: () => '/path/to/file' }
  })

  // an error, because path.basename was not defined
  await assert.rejects(async () => pathWrap.basename('/dog.png'), {
    name: 'TypeError',
    message: 'path.basename is not a function'
  })

  // use esmock.px to create a "partial mock"
  const pathWrapPartial = await esmock.px('../src/pathWrap.js', {
    path: { dirname: () => '/home/' }
  })

  // no error, because "core" path.basename was merged into the mock
  assert.deepEqual(pathWrapPartial.basename('/dog.png'), 'dog.png')
  assert.deepEqual(pathWrapPartial.dirname(), '/home/')
})