Creational Design Patterns

Design pattern adalah solusi efektif untuk masalah yang berulang dalam pengembangan perangkat lunak.

Sebuah pattern bukan kode yang langsung di-copy-paste — melainkan blueprint yang perlu disesuaikan dengan konteks spesifikmu. Satu pattern bisa memecahkan beberapa masalah, tapi tidak setiap masalah membutuhkan pattern.

Sumber: Refactoring Guru

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Singleton

Memastikan sebuah kelas hanya memiliki satu instansi, dan menyediakan titik akses global ke instansi tersebut.

Explorer

src

problem1-singleton

databaseConnection.before.ts

databaseConnection.ts

userRepository.before.ts

userRepository.ts

productRepository.before.ts

productRepository.ts

transactionRepository.before.ts

transactionRepository.ts

demo.ts

src/problem1-singleton/databaseConnection.ts

function sleep(ms: number) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

export class DatabaseConnection {
  private static maxConnectionPool: number = 2;
  private static currentConnectionPool: number = 0;
  private static _instance: DatabaseConnection | null = null;

  private constructor() {
    if (this.isConnectionFull()) throw Error("Connection pool is full");
    this.init();
  }

  // Intinya ProductRepository, TransactionRepository, UserRepository langsung ambil instance
  // tanpa harus buat instance baru lagi, karena sudah ada instance yang dibuat di constructor DatabaseConnection
  public static getInstance(): DatabaseConnection {
    if (this._instance === null) {
      this._instance = new DatabaseConnection();
    }
    return this._instance;
  }

  private async init() {
    console.log("Starting connection...");
    console.log("Connecting to database ...");
    console.log("✓ Connection created");
    this.increaseCurrentConnnectionPool();
  }

  private increaseCurrentConnnectionPool() {
    DatabaseConnection.currentConnectionPool++;
  }

  private isConnectionFull(): boolean {
    return (
      DatabaseConnection.currentConnectionPool ===
      DatabaseConnection.maxConnectionPool
    );
  }
}

Masalah — Setiap repository membuat koneksi baru

export class DatabaseConnection {
private static maxConnectionPool: number = 2;
private static currentConnectionPool: number = 0;

constructor() { // constructor PUBLIC — bisa dibuat berkali-kali
  if (this.isConnectionFull()) throw Error("Connection pool is full");
  this.init();
}
// ...
}

// ❌ Setiap repository membuat koneksi barunya sendiri
export class UserRepository {
private _db: DatabaseConnection;
constructor() {
  this._db = new DatabaseConnection(); // koneksi baru
}
}

export class ProductRepository {
private _db: DatabaseConnection;
constructor() {
  this._db = new DatabaseConnection(); // koneksi baru lagi
}
}

Solusi — Semua repository berbagi satu koneksi

export class DatabaseConnection {
private static _instance: DatabaseConnection | null = null;

private constructor() { // constructor PRIVATE — tidak bisa dibuat dari luar
  if (this.isConnectionFull()) throw Error("Connection pool is full");
  this.init();
}

public static getInstance(): DatabaseConnection {
  if (this._instance === null) {
    this._instance = new DatabaseConnection();
  }
  return this._instance; // selalu mengembalikan instansi yang sama
}
// ...
}

// ✅ Semua repository berbagi satu koneksi
export class UserRepository {
private _db: DatabaseConnection;
constructor() {
  this._db = DatabaseConnection.getInstance();
}
}

export class ProductRepository {
private _db: DatabaseConnection;
constructor() {
  this._db = DatabaseConnection.getInstance();
}
}

Kapan Digunakan

• Kamu butuh tepat satu instansi yang dibagi ke seluruh aplikasi (misal: koneksi DB, config, logger)
• Membuat objeknya membutuhkan proses yang mahal dan sebaiknya hanya dilakukan sekali

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Builder

Membangun objek yang kompleks secara bertahap, memisahkan proses konstruksi dari objek akhirnya.

Explorer

src

problem2-builder

iQueryBuilder.new.ts

query.ts

QueryBuilder.ts

demo.before.ts

demo.ts

src/problem2-builder/QueryBuilder.ts

// Ini file baru
import { IQueryBuilder } from "./iQueryBuilder.new";
import { Query } from "./query";

export class QueryBuilder implements IQueryBuilder {
  private _table: string;
  private _selectFields: string[];
  private _whereClauses: string[];

  constructor(table: string, selectFields: string[] = ["*"]) {
    this._table = table;
    this._selectFields = selectFields;
    this._whereClauses = [];
  }

  setTable(table: string): IQueryBuilder {
    this._table = table;
    return this;
  }

  setSelectFields(fields: string[]): IQueryBuilder {
    this._selectFields = fields;
    return this;
  }

  setWhereClauses(clauses: string[]): IQueryBuilder {
    this._whereClauses = clauses;
    return this;
  }

  get(): Query {
    return new Query(this._table, this._selectFields, this._whereClauses);
  }
}

Masalah — Constructor yang membingungkan

import { Query } from "./query";

// Susah dibaca — argumen apa saja ini?
const query = new Query("users", ["id", "username", "email"], []);

query.print();

// ❌ Object bisa dimodifikasi langsung dari luar
query.table = "";
query.print();

Solusi — Builder yang bisa di-chain

import { QueryBuilder } from "./QueryBuilder";

// Mudah dibaca — setiap langkah jelas tujuannya
const query = new QueryBuilder("users", ["id", "username", "email"])
.setWhereClauses(["age > 18"])
.get();

query.print();

// ✅ Object tidak bisa dimodifikasi sembarangan
// query.table = ""; // tidak perlu lagi

Kapan Digunakan

• Membangun objek dengan banyak konfigurasi opsional
• Kamu ingin proses konstruksi yang mudah dibaca, langkah demi langkah
• Proses konstruksi yang sama harus bisa menghasilkan representasi yang berbeda

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Prototype

Membuat objek baru dengan mengkloning objek yang sudah ada, daripada membangun dari awal.

Explorer

src

problem3-prototype

iClonable.new.ts

invoice.before.ts

invoice.ts

demo.before.ts

demo.ts

src/problem3-prototype/invoice.ts

import { IClonable } from "./iClonable.new";

export class Invoice implements IClonable {
  private _companyName: string;
  private _logo: string;
  private _footer: string;
  private _date: Date;
  private _clientName: string;
  private _items: string[];
  private _totalAmount: number;

  constructor(
    companyName: string, logo: string, footer: string,
    date: Date, clientName: string, items: string[], totalAmount: number,
  ) {
    console.log("Initializing new invoice...");
    console.log("Loading invoice layout template...");
    console.log("Loading company logo and footer...");
    console.log("Setting up styles and fonts...");
    console.log("✓ Invoice base setup complete.\n");

    this._companyName = companyName;
    this._logo = logo;
    this._footer = footer;
    this._date = date;
    this._clientName = clientName;
    this._items = items;
    this._totalAmount = totalAmount;
  }

  clone(): Invoice {
    return new Invoice(
      this._companyName, this._logo, this._footer,
      this._date, this._clientName, this._items, this._totalAmount,
    );
  }

  // Getters & Setters
  get clientName(): string { return this._clientName; }
  get items(): string[] { return this._items; }
  get totalAmount(): number { return this._totalAmount; }

  set clientName(value: string) { this._clientName = value; }
  set items(value: string[]) { this._items = value; }
  set totalAmount(value: number) { this._totalAmount = value; }

  public print() {
    console.log(`===== INVOICE FOR ${this._clientName} =====`);
    console.log(`Company: ${this._companyName}`);
    console.log(`Date: ${this._date.toDateString()}`);
    console.log(`Items: ${this._items.join(", ")}`);
    console.log(`Total: $${this._totalAmount}`);
    console.log(`${this._footer}`);
    console.log("=".repeat(100));
  }
}

Masalah — Setup mahal diulang terus

// Setiap invoice menjalankan constructor yang berat
const invoice1: Invoice = new Invoice(
"ABC Corp", "company_logo.png", "Thank you for your business!",
new Date(), "Client A", ["Laptop", "Mouse"], 1200,
);
// "Initializing new invoice..."
// "Loading invoice layout template..."
// "Loading company logo and footer..."
// "Setting up styles and fonts..."

const invoice2: Invoice = new Invoice(
"ABC Corp", "company_logo.png", "Thank you for your business!",
new Date(), "Client B", ["Monitor", "Keyboard"], 800,
);
// setup yang sama diulang lagi...

const invoice3: Invoice = new Invoice(
"ABC Corp", "company_logo.png", "Thank you for your business!",
new Date(), "Client C", ["Desk Chair"], 300,
);
// dan lagi...

Solusi — Clone lalu modifikasi

// Setup berat hanya dijalankan sekali
const invoice1: Invoice = new Invoice(
"ABC Corp", "company_logo.png", "Thank you for your business!",
new Date(), "Client A", ["Laptop", "Mouse"], 1200,
);

// Clone melewati constructor — hanya ubah yang berbeda
const invoice2: Invoice = invoice1.clone();
invoice2.clientName = "Client B";
invoice2.items = ["Monitor", "Keyboard"];
invoice2.totalAmount = 800;

const invoice3: Invoice = invoice1.clone();
invoice3.clientName = "Client C";
invoice3.items = ["Desk Chair"];
invoice3.totalAmount = 300;

Kapan Digunakan

• Pembuatan objek membutuhkan proses yang mahal (setup berat, panggilan DB, loading file)
• Kamu butuh banyak objek serupa yang hanya berbeda di beberapa properti
• Kamu ingin menghindari subclassing hanya untuk mendapatkan state awal yang berbeda

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Ringkasan

Pattern	Tujuan	Mekanisme Utama
Singleton	Hanya satu instansi, dibagi secara global	Constructor private + getInstance()
Builder	Membangun objek kompleks secara bertahap	Method berantai + get()
Prototype	Mengkloning objek yang ada daripada membuatnya	Method clone()