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name: factory-function-composition description: Apply factory function patterns to compose clients and services with proper separation of concerns. Use when creating functions that depend on external clients, wrapping resources with domain-specific methods, or refactoring code that mixes client/service/method options together. metadata: author: epicenter version: '1.0'

Factory Function Composition

This skill helps you apply factory function patterns for clean dependency injection and function composition in TypeScript.

When to Apply This Skill

Use this pattern when you see:

• A function that takes a client/resource as its first argument
• Options from different layers (client, service, method) mixed together
• Client creation happening inside functions that shouldn't own it
• Functions that are hard to test because they create their own dependencies

The Universal Signature

Every factory function follows this signature:

Copyfunction createSomething(dependencies, options?) {
	return {
		/* methods */
	};
}

• First argument: Always the resource(s). Either a single client or a destructured object of multiple dependencies.
• Second argument: Optional configuration specific to this factory. Never client config—that belongs at client creation.

Two arguments max. First is resources, second is config. No exceptions.

The Core Pattern

Copy// Single dependency
function createService(client, options = {}) {
	return {
		method(methodOptions) {
			// Uses client, options, and methodOptions
		},
	};
}

// Multiple dependencies
function createService({ db, cache }, options = {}) {
	return {
		method(methodOptions) {
			// Uses db, cache, options, and methodOptions
		},
	};
}

// Usage
const client = createClient(clientOptions);
const service = createService(client, serviceOptions);
service.method(methodOptions);

Key Principles

1. Client configuration belongs at client creation time — don't pipe clientOptions through your factory
2. Each layer has its own options — client, service, and method options stay separate
3. Dependencies come first — factory functions take dependencies as the first argument
4. Return objects with methods — not standalone functions that need the resource passed in

Recognizing the Anti-Patterns

Anti-Pattern 1: Function takes client as first argument

Copy// Bad
function doSomething(client, options) { ... }
doSomething(client, options);

// Good
const service = createService(client);
service.doSomething(options);

Anti-Pattern 2: Client creation hidden inside

Copy// Bad
function doSomething(clientOptions, methodOptions) {
	const client = createClient(clientOptions); // Hidden!
	// ...
}

// Good
const client = createClient(clientOptions);
const service = createService(client);
service.doSomething(methodOptions);

Anti-Pattern 3: Mixed options blob

Copy// Bad
doSomething({
	timeout: 5000, // Client option
	retries: 3, // Client option
	endpoint: '/users', // Method option
	payload: data, // Method option
});

// Good
const client = createClient({ timeout: 5000, retries: 3 });
const service = createService(client);
service.doSomething({ endpoint: '/users', payload: data });

Anti-Pattern 4: Multiple layers hidden

Copy// Bad
function doSomething(clientOptions, serviceOptions, methodOptions) {
	const client = createClient(clientOptions);
	const service = createService(client, serviceOptions);
	return service.method(methodOptions);
}

// Good — each layer visible and configurable
const client = createClient(clientOptions);
const service = createService(client, serviceOptions);
service.method(methodOptions);

Multiple Dependencies

When your service needs multiple clients:

Copyfunction createService(
	{ db, cache, http }, // Dependencies as destructured object
	options = {}, // Service options
) {
	return {
		method(methodOptions) {
			// Uses db, cache, http
		},
	};
}

// Usage
const db = createDbConnection(dbOptions);
const cache = createCacheClient(cacheOptions);
const http = createHttpClient(httpOptions);

const service = createService({ db, cache, http }, serviceOptions);
service.method(methodOptions);

The Mental Model

Think of it as a chain where each link:

• Receives a resource from the previous link
• Adds its own configuration
• Produces something for the next link

CopycreateClient(...)  →  createService(client, ...)  →  service.method(...)
     ↑                       ↑                            ↑
 clientOptions          serviceOptions              methodOptions

Benefits

• Testability: Inject mock clients easily
• Reusability: Share clients across multiple services
• Flexibility: Configure each layer independently
• Clarity: Clear ownership of configuration at each level

References

See the full articles for more details:

• The Universal Factory Function Signature — signature explained in depth
• Stop Passing Clients as Arguments — practical guide
• The Factory Function Pattern — detailed explanation
• Factory Method Patterns — separating options and method patterns