---
name: simple-di
description: "A practical pattern for dependency injection in TypeScript using factory functions, plain objects, and zero frameworks."
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---

## Simple DI: Factory-Based Dependency Injection in TypeScript

Implement framework-less dependency injection using factory functions, typed deps objects, and a Proxy-based registry. No decorators, no reflect-metadata, no framework.

## Signals to apply

- Test files use jest.mock or vi.mock for multiple dependencies per test
- Services import other services directly at the module level
- Circular dependencies cause runtime errors or slow builds
- Adding a dependency to a service requires updating unrelated test mocks

## Implementation steps

1. For each service, define a deps type listing its dependencies as an object:
   type UserServiceDeps = { db: Database; logger: Logger; emailService: EmailService }

2. Convert the service to a factory function that takes deps and returns an object of methods:
   export const createUserService = (deps: UserServiceDeps) => ({
     getUser: async (id: string) => deps.db.users.findOne({ id }),
     createUser: async (data: NewUser) => { ... },
   })

3. Create a factory.ts utility (~25 lines) with a Proxy-based registry:
   - factory(init) registers a lazy initializer, returns a Proxy that throws if accessed before instantiation
   - factory.instantiateAll() calls all registered initializers
   - factory.initializeAll() calls onInit() on services that define it (for post-instantiation setup)
   - Use Object.assign to merge static methods onto the create function

4. In each service file, wrap creation with factory():
   export const userService = factory(() => createUserService({ db, logger, emailService }))

5. At the app entry point, boot with two calls:
   factory.instantiateAll()
   factory.initializeAll()

6. In tests, call the factory function directly with minimal fakes. Only implement methods the test exercises:
   const svc = createUserService({ db: fakeDb as Database, logger: { info: () => {} } as Logger })

7. For circular dependencies: both files import each other's proxy export. The Proxy resolves the cycle because modules export the proxy at load time, not the real instance.

## Key constraints

- Never call dependencies during construction (inside the factory initializer). Use onInit() for setup that needs deps.
- Adopt incrementally: convert one painful-to-test service first as proof of concept
- The Proxy get handler must bind functions to the instance (value.bind(instance)) to preserve this context