typescript 59 lines · 7 steps

Decoupling side effects with NestJS events

Placing an order emits a single event, letting inventory, email, and logging react independently without the service knowing about them.

Explained by highlit
NestJS Intermediate event-driven decoupling dependency-injection pub-sub side-effects
1import { Injectable, NotFoundException } from '@nestjs/common';
2import { EventEmitter2, OnEvent } from '@nestjs/event-emitter';
3import { InjectRepository } from '@nestjs/typeorm';
4import { Repository } from 'typeorm';
5import { Order } from './order.entity';
6import { MailerService } from '../mailer/mailer.service';
7import { InventoryService } from '../inventory/inventory.service';
8 
9export class OrderPlacedEvent {
10 constructor(
11 public readonly orderId: string,
12 public readonly customerEmail: string,
13 public readonly items: { sku: string; quantity: number }[],
14 ) {}
15}
16 
17@Injectable()
18export class OrdersService {
19 constructor(
20 @InjectRepository(Order) private readonly orders: Repository<Order>,
21 private readonly eventEmitter: EventEmitter2,
22 ) {}
23 
24 async placeOrder(customerEmail: string, items: { sku: string; quantity: number }[]) {
25 const order = await this.orders.save(
26 this.orders.create({ customerEmail, items, status: 'pending' }),
27 );
28 
29 this.eventEmitter.emit(
30 'order.placed',
31 new OrderPlacedEvent(order.id, customerEmail, items),
32 );
33 
34 return order;
35 }
36}
37 
38@Injectable()
39export class OrderNotificationsListener {
40 constructor(
41 private readonly mailer: MailerService,
42 private readonly inventory: InventoryService,
43 ) {}
44 
45 @OnEvent('order.placed', { async: true })
46 async onOrderPlaced(event: OrderPlacedEvent) {
47 await this.inventory.reserve(event.items);
48 await this.mailer.send({
49 to: event.customerEmail,
50 template: 'order-confirmation',
51 context: { orderId: event.orderId },
52 });
53 }
54 
55 @OnEvent('order.*')
56 logOrderActivity(event: OrderPlacedEvent) {
57 console.log(`[orders] activity for ${event.orderId}`);
58 }
59}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Emitting a domain event keeps the core write path unaware of downstream consumers, so adding reactions never touches the service.
  2. 2A typed event class carries exactly the data listeners need, avoiding refetches and keeping handlers self-contained.
  3. 3Wildcard subscriptions let cross-cutting concerns like logging hook into whole families of events at once.

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