How does the NestJS bootstrap sequence work?

Q: How does the NestJS bootstrap sequence work?

1. NestFactory.create(AppModule) is called. 2. NestJS recursively scans the module graph — resolves all imports, providers, exports. 3. The IoC container is built; all singleton providers are instantiated. 4. OnModuleInit lifecycle hook fires for each module (depth-first order). 5. OnApplicationBootstrap fires for each module. 6. app.listen() starts the HTTP server — app is now ready to accept requests. @Injectable() export class DatabaseService implements OnModuleInit, OnModuleDestroy { async onModuleInit() { // Runs after all providers are instantiated await this.connect(); console.log('Database connected'); } async onModuleDestroy() { // Runs on graceful shutdown await this.disconnect(); console.log('Database disconnected'); } }

Difficulty: 5/10

Application bootstrap, Lifecycle hooks, Module initialization

NestJS bootstrap: NestFactory.create() scans the module graph, builds the DI container, instantiates all singletons, fires OnModuleInit hooks (depth-first), fires OnApplicationBootstrap hooks, then starts the HTTP server. On shutdown: OnModuleDestroy, BeforeApplicationShutdown, then OnApplicationShutdown fire in order.

Full bootstrap sequence:

1
1. NestFactory.create(AppModule) is called.
2
1. NestJS recursively scans the module graph — resolves all imports, providers, exports.
3
1. The IoC container is built; all singleton providers are instantiated.
4
1. OnModuleInit lifecycle hook fires for each module (depth-first order).
5
1. OnApplicationBootstrap fires for each module.
6

Implementing lifecycle hooks

Scenario Questions

0-2 years experience

2-5 years experience

1We added a custom logger that must be initialized before any other providers, but during tests it sometimes logs after the first request is handled. How would you adjust the bootstrap sequence to guarantee the logger is ready?

Follow-up Questions