Hey HN!

Suites is a unit-testing framework for TypeScript backend systems working with dependency injection.

It introduces a single declarative API for unit testing: wrap any unit with a single call and receive a correct test environment for the unit and its dependencies.

  // Test a class in complete isolation, all dependencies mocked
  TestBed.solitary(OrderService).compile();

  // Or, test how it integrates with a real dependency
  TestBed.sociable(OrderService)
    .expose(PaymentProcessor) // Use the real class
    .compile();

The backstory: while working for a fast-growing fintech startup 4 years ago, my team had to write thousands of unit tests for our NestJS backend. To scale our efforts, I've built a unit-testing framework which eliminated a significant part of the tests' code and made them a whole lot more maintainable. I open-sourced it, generalized it to support more tech stacks, and soon it gained traction.

The problem - unit testing becomes a nightmare very fast:

- You mock dependencies by hand, wire them up manually, and spend more time maintaining tests than writing them. - Unit tests are leaky and error-prone since it's unclear which dependencies are mocked - Test intention is lost in the boilerplate - Each engineer invents their own conventions - Tests break during refactors because of untyped, loosely connected mocks - Missing mock implementations go unnoticed, so tests falsely fail on unexpected undefined returns or validate the wrong behavior - Test errors for wrong mocking are confusing to engineers and coding agents alike

The nitty-gritty details:

- Fluent, auto-discoverable, fully-typed API. - DI Framework Adapters: Works out of the box with NestJS and InversifyJS - Testing Library Adapters: Works out of the box with Jest, Sinon, and Vitest

Suites provide two modes of testing: (inspired by Martin Fowler's writings [0])

- Solitary: For testing units in a vacuum, it creates type-safe mocks of all the unit's direct dependencies. - Sociable: For testing units with other related units, it allows the developer to define which dependencies to mock.

Eliminating boilerplate and making mocks type-safe drastically helps coding agents (e.g. Claude Code) to write correct tests in a single pass and avoid entering infinite loops, significantly reducing token cost.

You can read more about Suites on our website: https://suites.dev

What's next?

- A dedicated skill for Claude Code - Extended docs with real-world patterns - More test utilities for common backend scenarios

Suites is fully open-source and actively maintained for 4 years. I would love you to check it out and get your honest feedback!

[0] https://martinfowler.com/bliki/UnitTest.html