@rupertsworld/dependencies

Lazy singleton dependency container with stable proxies, typed identifiers, and optional class-based injection.

Install

npm install @rupertsworld/dependencies

Why

Use this when one part of your app is responsible for wiring services up at runtime, but other parts of the app need access to those services without importing back into the entry point.

A common case is:

• index.ts imports UI modules
• index.ts creates services later during startup
• the UI modules need those services
• the UI cannot import back from index.ts without creating a circular import

In that setup, both sides import the container instead. The bootstrap code registers services, and the rest of the app resolves them when needed.

It also helps with circular service wiring. resolve(...) returns a stable lazy proxy, so two services can hold references to each other without forcing immediate construction order. The real instance is only created on first real use.

For design rationale and architectural notes, see docs/architecture.md.

Runnable example

A tiny Vite app lives in example/ and demonstrates the real entry-point pattern this package is meant for.

Run it with:

cd dependencies/example
npm install
npm run dev

Usage

register(Class)

Use register(Class) when the container can fully construct the class itself.

import { dependencies } from "@rupertsworld/dependencies";

class Database {
  read() {
    return "db";
  }
}

dependencies.register(Database);

const db = dependencies.resolve(Database);
db.read();

This also works for self-resolving classes:

class Editor {
  db = dependencies.resolve(Database);

  render() {
    return this.db.read();
  }
}

dependencies.register(Editor);

register(Key, () => value)

Use a factory when you need custom wiring, runtime args, or an existing instance.

const db = await Database.connect(process.env.DATABASE_URL!);
dependencies.register(Database, () => db);

dependencies.register("appDb", () => db);

Factories must be synchronous. If setup is async, await it before registering.

@injectable(...)

Use @injectable(...) when you want constructor injection with register(Class).

import { dependencies, injectable } from "@rupertsworld/dependencies";

class Database {
  read() {
    return "db";
  }
}

class Logger {
  format(value: string) {
    return `log:${value}`;
  }
}

@injectable(Database, Logger)
class Editor {
  constructor(
    readonly db: Database,
    readonly logger: Logger,
  ) {}

  render() {
    return this.logger.format(this.db.read());
  }
}

dependencies.register(Database);
dependencies.register(Logger);
dependencies.register(Editor);

dependencies.resolve(Editor).render();

createIdentifier<T>(name)

Use identifiers for interfaces or multiple instances of the same class.

import {
  createIdentifier,
  dependencies,
} from "@rupertsworld/dependencies";

interface AuthService {
  check(): boolean;
}

const IAuth = createIdentifier<AuthService>("IAuth");

dependencies.register(IAuth, () => ({
  check: () => true,
}));

const auth = dependencies.resolve(IAuth);
auth.check();

String keys still work as a fallback:

const db = dependencies.resolve<Database>("appDb");

Circular construction

This is supported as long as constructors only store references and do not call back into each other during construction.

dependencies.register(Database, () => new Database(dependencies.resolve(Logger)));
dependencies.register(Logger, () => new Logger(dependencies.resolve(Database)));

If you create a true behavioral cycle during construction, the container throws with the full path:

Circular dependency detected: Database -> Logger -> Database

Notes

• resolve(key) returns one stable proxy per key.
• Nothing is instantiated on register(...) or resolve(...); instantiation happens on first real property access.
• If a key has not been registered yet, using its proxy throws until you register it.
• Re-registering a key keeps existing proxies stable and forwards them to the new instance on future access.
• Proxies are not promise-like and should not be relied on for full reflection parity such as instanceof.

API

dependencies

Exported singleton DependencyContainer instance.

DependencyContainer

Method	Description
register(Class)	Register a class for container-managed construction.
register(key, () => value)	Register a manual factory for a class key, identifier, or string key.
resolve(key)	Resolve a stable lazy proxy for the key.
has(key)	Return whether the key is currently registered.

createIdentifier<T>(name)

Create a typed identifier for interfaces or multiple instances of the same class.

@injectable(...dependencies)

Declare constructor dependencies for classes that will be registered with register(Class).