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