Extend Jest expect matchers for native Node.js test runner mock objects

Extend the Jest expect library with matchers that work with native Node.js test runner mock objects.

The native Node.js test runner has a different mock structure from jest.mock, therefore extending the current methods is necessary to provide Jest-like testing experience.

Why use this?

The native Node.js test runner (introduced in Node.js 18) provides built-in testing capabilities without external dependencies. However, its mock objects have a different structure than Jest mocks, making Jest's expect matchers incompatible.

This package bridges that gap by:

• ✅ Providing familiar Jest-like assertions for Node.js mocks
• ✅ Zero configuration setup
• ✅ Lightweight with minimal dependencies
• ✅ Full compatibility with existing Jest expect patterns
• ✅ Perfect for migrating from Jest to native Node.js testing

Installation

npm install expect-matcher-node-mock

yarn add expect-matcher-node-mock

pnpm add expect-matcher-node-mock

Usage

Since the native Node.js test runner doesn't have any global setup, you need to import this extension in every test file:

import 'expect-matcher-node-mock';

The extension of expect matchers is included in this import.

Alternative: Import expect directly

You can also import expect directly from this package, which automatically includes all the extended matchers:

import { expect } from 'expect-matcher-node-mock';

JavaScript Example

import { test, mock } from 'node:test';
import { expect } from 'expect-matcher-node-mock'; // Direct import with matchers included

test('mock function testing', () => {
  const mockFn = mock.fn();
  
  mockFn('arg1', 'arg2');
  mockFn('arg3');
  
  expect(mockFn).toHaveBeenCalled();
  expect(mockFn).toHaveBeenCalledTimes(2);
  expect(mockFn).toHaveBeenCalledWith('arg1', 'arg2');
  expect(mockFn).toHaveBeenLastCalledWith('arg3');
});

Alternative Import Method

import { test, mock } from 'node:test';
import { expect } from 'expect';
import 'expect-matcher-node-mock'; // Extension import

test('same functionality as above', () => {
  const mockFn = mock.fn();
  // ... same test code
});

TypeScript Example

import { test, mock } from 'node:test';
import { expect } from 'expect-matcher-node-mock'; // Direct import recommended

test('mock function with return values', () => {
  const mockFn = mock.fn<(x: number) => number>();
  
  mockFn.mock.mockImplementation((x: number) => x * 2);
  
  const result1 = mockFn(5);
  const result2 = mockFn(10);
  
  expect(mockFn).toHaveBeenCalledTimes(2);
  expect(mockFn).toHaveReturnedWith(10);
  expect(mockFn).toHaveLastReturnedWith(20);
});

Advanced Example - Method Mocking

import { test, mock } from 'node:test';
import { expect } from 'expect-matcher-node-mock';

test('object method mocking', () => {
  const obj = {
    method: mock.fn(() => 'original'),
    calculate: mock.fn((a, b) => a + b)
  };
  
  obj.method();
  obj.calculate(2, 3);
  obj.calculate(5, 7);
  
  expect(obj.method).toHaveBeenCalled();
  expect(obj.calculate).toHaveBeenCalledTimes(2);
  expect(obj.calculate).toHaveBeenNthCalledWith(1, 2, 3);
  expect(obj.calculate).toHaveBeenNthCalledWith(2, 5, 7);
  expect(obj.calculate).toHaveNthReturnedWith(2, 12);
});

Available Matchers

toHaveBeenCalled

https://jestjs.io/docs/expect#tohavebeencalled

toHaveBeenCalledTimes

https://jestjs.io/docs/expect#tohavebeencalledtimesnumber

toHaveBeenCalledWith

https://jestjs.io/docs/expect#tohavebeencalledwitharg1-arg2-

toHaveBeenLastCalledWith

https://jestjs.io/docs/expect#tohavebeenlastcalledwitharg1-arg2-

toHaveBeenNthCalledWith

https://jestjs.io/docs/expect#tohavebeennthcalledwithnthcall-arg1-arg2-

toHaveReturned

Alias: toReturn
https://jestjs.io/docs/expect#tohavereturned

toHaveReturnedTimes

https://jestjs.io/docs/expect#tohavereturnedtimesnumber

toHaveReturnedWith

https://jestjs.io/docs/expect#tohavereturnedwithvalue

toHaveLastReturnedWith

https://jestjs.io/docs/expect#tohavelastreturnedwithvalue

toHaveNthReturnedWith

https://jestjs.io/docs/expect#tohaventhreturnedwithnthcall-value

Requirements

• Node.js 18.0.0 or higher (for native test runner support)
• expect package (peer dependency) - Install with npm install expect

Peer Dependencies

This package requires the expect library to be installed in your project:

npm install expect

Troubleshooting

"expect is not defined" error

You have two options to import expect:

Option 1: Direct import (recommended)

import { expect } from 'expect-matcher-node-mock';

Option 2: Separate imports

import { expect } from 'expect';
import 'expect-matcher-node-mock';

Matchers not working

Ensure you either:

• Import expect directly from this package: import { expect } from 'expect-matcher-node-mock';
• Or import the extension before using expect: import 'expect-matcher-node-mock';

TypeScript support

While this package is written in JavaScript, it works seamlessly with TypeScript projects. For better type support, you may want to add type declarations for the extended matchers.

Compatibility

• ✅ Node.js native test runner (18.0.0+)
• ✅ Works with ES modules (.mjs, type: "module")
• ✅ TypeScript projects
• ✅ CommonJS projects (with appropriate imports)

License

MIT - see LICENSE file for details.

Contributing

Issues and pull requests are welcome on GitHub.

Development

# Clone the repository
git clone https://github.com/crysadrak/expect-matcher-node-mock.git
cd expect-matcher-node-mock

# Install dependencies
npm install

# Run tests
npm test

# Run linting
npm run lint

Changelog

See GitHub Releases for version history.

Related Projects

• expect - The core expect library
• Node.js Test Runner - Native Node.js testing
• Jest - JavaScript testing framework