Observer Pattern

1// Event types
2type EventMap = {
3  userCreated: { userId: number; email: string };
4  userDeleted: { userId: number };
5  orderPlaced: { orderId: string; total: number };
6};
7 
8// Generic typed event emitter
9class TypedEventEmitter<T extends Record<string, unknown>> {
10  private listeners: {
11    [K in keyof T]?: Array<(data: T[K]) => void>;
12  } = {};
13 
14  on<K extends keyof T>(event: K, callback: (data: T[K]) => void): () => void {
15    if (!this.listeners[event]) {
16      this.listeners[event] = [];
17    }
18    this.listeners[event]!.push(callback);
19 
20    // Return unsubscribe function
21    return () => this.off(event, callback);
22  }
23 
24  off<K extends keyof T>(event: K, callback: (data: T[K]) => void): void {
25    const callbacks = this.listeners[event];
26    if (callbacks) {
27      const index = callbacks.indexOf(callback);
28      if (index > -1) {
29        callbacks.splice(index, 1);
30      }
31    }
32  }
33 
34  emit<K extends keyof T>(event: K, data: T[K]): void {
35    const callbacks = this.listeners[event];
36    if (callbacks) {
37      callbacks.forEach((callback) => callback(data));
38    }
39  }
40 
41  once<K extends keyof T>(event: K, callback: (data: T[K]) => void): void {
42    const unsubscribe = this.on(event, (data) => {
43      unsubscribe();
44      callback(data);
45    });
46  }
47}
48 
49// Usage with application events
50const events = new TypedEventEmitter<EventMap>();
51 
52// Subscribe to events
53events.on("userCreated", ({ userId, email }) => {
54  console.log(`New user: ${email} (ID: ${userId})`);
55});
56 
57events.on("userCreated", ({ email }) => {
58  console.log(`Sending welcome email to ${email}`);
59});
60 
61const unsubscribeOrder = events.on("orderPlaced", ({ orderId, total }) => {
62  console.log(`Order ${orderId}: $${total.toFixed(2)}`);
63});
64 
65// One-time subscription
66events.once("userDeleted", ({ userId }) => {
67  console.log(`User ${userId} deleted (one-time notification)`);
68});
69 
70// Emit events
71events.emit("userCreated", { userId: 1, email: "alice@example.com" });
72events.emit("orderPlaced", { orderId: "ORD-001", total: 99.99 });
73events.emit("userDeleted", { userId: 1 });
74events.emit("userDeleted", { userId: 2 }); // Won't trigger once handler
75 
76// Unsubscribe from order events
77unsubscribeOrder();
78events.emit("orderPlaced", { orderId: "ORD-002", total: 149.99 }); // Not logged