Design Patterns

1// Module Pattern (encapsulation)
2const UserModule = (() => {
3  let users = []; // Private
4  return {
5    add(user) { users.push(user); },
6    getAll() { return [...users]; },
7    findById(id) { return users.find(u => u.id === id); }
8  };
9})();
10
11// Singleton Pattern
12class Database {
13  constructor() {
14    if (Database.instance) return Database.instance;
15    this.connection = "connected";
16    Database.instance = this;
17  }
18  static getInstance() {
19    if (!Database.instance) new Database();
20    return Database.instance;
21  }
22}
23
24// Observer Pattern (Event Emitter)
25class EventEmitter {
26  constructor() { this.events = {}; }
27  on(event, listener) {
28    (this.events[event] ||= []).push(listener);
29    return () => this.off(event, listener); // Return unsubscribe
30  }
31  off(event, listener) {
32    this.events[event] = this.events[event]?.filter(l => l !== listener);
33  }
34  emit(event, ...args) {
35    this.events[event]?.forEach(listener => listener(...args));
36  }
37}
38
39// Factory Pattern
40class Shape {
41  static create(type, ...args) {
42    switch (type) {
43      case "circle": return new Circle(...args);
44      case "square": return new Square(...args);
45      case "triangle": return new Triangle(...args);
46      default: throw new Error(`Unknown shape: ${type}`);
47    }
48  }
49}
50
51// Strategy Pattern
52class Sorter {
53  constructor(strategy) { this.strategy = strategy; }
54  sort(data) { return this.strategy(data); }
55  setStrategy(strategy) { this.strategy = strategy; }
56}
57const bubbleSort = (data) => { /* ... */ return data; };
58const quickSort = (data) => { /* ... */ return data; };
59const sorter = new Sorter(quickSort);
60
61// Pub/Sub Pattern
62class PubSub {
63  constructor() { this.topics = {}; }
64  subscribe(topic, handler) {
65    (this.topics[topic] ||= []).push(handler);
66    return { unsubscribe: () => {
67      this.topics[topic] = this.topics[topic].filter(h => h !== handler);
68    }};
69  }
70  publish(topic, data) {
71    this.topics[topic]?.forEach(handler => handler(data));
72  }
73}