NestJS Basic

NestJS has many decorators, so it is easy to write syntactically correct code while misunderstanding when things happen. Two questions often get mixed up: when the application is assembled, and what a request passes through.

Separating these two questions is the fastest way to read Nest errors.

Two different moments

Moment	What happens	Read
Bootstrap	Nest reads module metadata, builds the graph, creates providers. Modules are only declarations and do not rerun per request	Modules, providers, controllers, DI container
A request arrives	The request passes through middleware, guards, interceptors, pipes to the handler, then back out through interceptors; errors branch off to filters	Request lifecycle

The nastiest bugs live where the two meet: storing one request’s state in a field of a default provider (shared by every request), or putting request scope on a central provider so the whole tree is recreated per request (provider scopes).

Reading path

Order	Article	Level	Answers
1	Modules, providers and controllers	Basic	The three roles, import/export, reading “can’t resolve dependencies” errors
2	DI container	Intermediate	Tokens, useClass vs useExisting, @Optional, async providers, dynamic modules
3	Provider scopes	Intermediate	How many instances, why request scope bubbles up, REQUEST or ALS
4	Request lifecycle	Intermediate	Layer order, the three paths, which errors filters catch
5	Custom decorators & Reflector	Intermediate	@Public(), @Roles(), override or merge metadata
6	Dynamic modules	Intermediate	forRoot/forRootAsync, why importing twice yields two instances
7	DTO validation	Basic	whitelist, nested DTOs, query params, mass assignment
8	Exception filters & error responses	Intermediate	One error format, mapping 23505 to 409, errors that never reach filters
9	Cross-service transactions (CLS)	Advanced	Keeping one connection across services, propagation, the forgotten-await trap

Pick an article by situation

Situation	Read
New to NestJS	Article 1, then article 4
UnknownDependenciesException at startup	Modules, providers and controllers, the import/export section
Need two tokens pointing to the same instance	DI container, the useExisting section
Unsure whether logic belongs in a guard, interceptor or pipe	Request lifecycle
Two requests overwrite each other’s currentTenant	Provider scopes, then AsyncLocalStorage
@Public() on a controller has no effect	Custom decorators & Reflector
A client sends role: "admin" and it gets through	DTO validation
Every error has a different format, DB errors become 500	Exception filters & error responses
A failure midway leaves half-written data across services	Cross-service transactions (CLS)
Suspect a handler is slowing other requests	No dedicated NestJS article yet; see Node.js event loop

This section currently has the nine articles above. There are no articles yet on testing, microservices or GraphQL in NestJS.