Creational Patterns

// Singleton — one instance shared across the app
class Database { private static instance: Database; private constructor(private url: string) {} static getInstance(): Database { if (!Database.instance) { Database.instance = new Database(process.env.DATABASE_URL!); } return Database.instance; } query(sql: string) { /* ... */ }
}

const db1 = Database.getInstance();
const db2 = Database.getInstance();
console.log(db1 === db2); // true

// Factory Method — delegate instantiation to subclasses
interface Button { render(): string; }

class PrimaryButton implements Button { render() { return '<button class="btn-primary">Click</button>'; }
}
class DangerButton implements Button { render() { return '<button class="btn-danger">Delete</button>'; }
}

function createButton(type: 'primary' | 'danger'): Button { if (type === 'primary') return new PrimaryButton(); return new DangerButton();
}

// Builder — construct complex objects step by step
class QueryBuilder { private table = ''; private conditions: string[] = []; private limitVal: number | null = null; from(table: string) { this.table = table; return this; } where(condition: string) { this.conditions.push(condition); return this; } limit(n: number) { this.limitVal = n; return this; } build(): string { let sql = \`SELECT * FROM \${this.table}\`; if (this.conditions.length) sql += \` WHERE \${this.conditions.join(' AND ')}\`; if (this.limitVal !== null) sql += \` LIMIT \${this.limitVal}\`; return sql; }
}

const query = new QueryBuilder() .from('users') .where('active = true') .where('age > 18') .limit(10) .build();
// SELECT * FROM users WHERE active = true AND age > 18 LIMIT 10