Singleton Pros & Cons

Introduction

Singletons are a controversial pattern. They’re useful in moderation but can create problems when overused.

When to Use Singletons

Configuration: App-wide settings that are consistent everywhere
Logging: A centralized logger for the application
Database connections: A single connection pool
Caching: A shared cache instance

When to Avoid Singletons

// ❌ Bad — singleton makes testing difficult
class PaymentService {
  processPayment(amount) {
    Logger.getInstance().log('Processing payment...');
    Database.getInstance().query('INSERT INTO payments...');
  }
}

// ✅ Better — dependency injection for testability
class PaymentService {
  constructor(logger, database) {
    this.logger = logger;
    this.database = database;
  }

  processPayment(amount) {
    this.logger.log('Processing payment...');
    this.database.query('INSERT INTO payments...');
  }
}

// Now you can mock in tests!
const mockLogger = { log: jest.fn() };
const mockDb = { query: jest.fn() };
const service = new PaymentService(mockLogger, mockDb);

Drawbacks

Hidden dependencies: Components implicitly depend on the singleton
Testing difficulty: Hard to mock or replace in tests
Global state: Creates implicit global state
Tight coupling: Components become coupled to the singleton

Summary

Pros	Cons
Single instance guarantee	Global state
Easy access	Hidden dependencies
Consistent state	Hard to test
Saves memory	Tight coupling

Singleton Pros & Cons

Singleton Pros & Cons

Introduction

When to Use Singletons

When to Avoid Singletons

Drawbacks

Summary

Further Reading