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:

typescript

function 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

typescript

// 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

Client configuration belongs at client creation time — don't pipe clientOptions through your factory
Each layer has its own options — client, service, and method options stay separate
Dependencies come first — factory functions take dependencies as the first argument
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

typescript

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

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

Anti-Pattern 2: Client creation hidden inside

typescript

// 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

typescript

// 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

typescript

// 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:

typescript

function 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

createClient(...)  →  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

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Metadata

SKILL.md

Factory Function Composition

When to Apply This Skill

The Universal Signature

The Core Pattern

Key Principles

Recognizing the Anti-Patterns

Anti-Pattern 1: Function takes client as first argument

Anti-Pattern 2: Client creation hidden inside

Anti-Pattern 3: Mixed options blob

Anti-Pattern 4: Multiple layers hidden

Multiple Dependencies

The Mental Model

Benefits

References