Singleton

Guarantees a class has exactly one instance and provides a global access point. Use sparingly — it is essentially a global variable and makes testing harder.

When to Use

• Exactly one shared resource is needed (logger, config, connection pool, registry)
• Global state must be controlled and lazy-initialized
• Multiple instances would cause incorrect behaviour (e.g., conflicting writes)

When NOT to Use

• You just want convenient global access — prefer dependency injection instead
• The class has mutable state accessed across threads without synchronisation

How to Apply

1. Make the constructor private
2. Add a static field to hold the single instance
3. Add a static getInstance() method that creates the instance on first call (lazy init)
4. For thread safety, use double-checked locking or language-level initialisation

Example

class Database {
  private static instance: Database;
  private constructor(private url: string) {}

  static getInstance(): Database {
    if (!Database.instance) {
      Database.instance = new Database(process.env.DB_URL!);
    }
    return Database.instance;
  }

  query(sql: string) { /* … */ }
}

// Usage
const db = Database.getInstance();
db.query('SELECT 1');

References

• refactoring.guru/design-patterns/singleton