Debug and Optimize Nest.js Applications

Nest.js Expert is a debugging and architecture specialist for Nest.js applications, covering module architecture and dependency injection, controllers and request handling, middleware/guards/interceptors/pipes, testing with Jest and Supertest, database integration via TypeORM and Mongoose, authentication and authorization with Passport.js, configuration/environment management, and error handling/logging. It runs an environmental-adaptation flow first: a detection phase that checks the Nest.js version, database setup, authentication configuration, and module structure, followed by adaptation strategies and diagnostic tooling that analyzes module dependencies, checks for circular dependencies, validates module structure, and verifies fixes in a defined order. Its core value is a library of 17 named, real-world problems sourced from GitHub issues and Stack Overflow with their fixes - among them "Nest can't resolve dependencies of the [Service]," circular dependency detection, e2e test dependency resolution failures, TypeOrmModule connection failures, unknown Passport strategy errors, a module exporting itself instead of its service, missing JWT secretOrPrivateKey, version-specific regressions, 401 Unauthorized with missing Passport JWT credentials, production memory leaks, multiple database connections, and misleading "unable to connect" errors that mask the real TypeORM failure. Reusable code patterns include module organization, a custom decorator pattern, a testing pattern, and an exception filter pattern, plus quick-reference snippets for dependency injection tokens, and the Global Module pattern:

@Global()
@Module({
  providers: [GlobalService],
  exports: [GlobalService],
})
export class GlobalModule {}

and the Dynamic Module pattern (a static forRoot() factory returning a DynamicModule).

Use this skill when diagnosing or building Nest.js module architecture, DI, controllers, middleware/guards/pipes, testing, database integration, authentication, or configuration - especially when hitting one of its named real-world error patterns.

Given a Nest.js problem or design question, the skill outputs a diagnosis matched to its problem library where applicable, plus a code review checklist covering module architecture/DI, testing and mocking, TypeORM-focused database integration, JWT/Passport authentication and security, request lifecycle/middleware, and performance/optimization. For architecture decisions it provides six decision trees: choosing a database ORM, module organization strategy, testing strategy selection, authentication method, caching strategy, and performance optimization approach, plus concrete caching and database optimization techniques for request processing. A fix or architecture change is validated against nine named success criteria: the problem is correctly located in module structure, the solution follows Nest.js architectural patterns, all unit/integration/e2e tests pass, no circular dependencies are introduced, performance metrics are maintained or improved, project conventions are followed, error handling is proper, security best practices are applied, and API-change documentation is updated.

Built on the Nest.js framework with TypeORM and Mongoose for database access, Jest and Supertest for testing, and Passport.js for authentication. Points to specific external resources by category: Nest.js Documentation, CLI docs, and Recipes for core documentation; Jest docs, Supertest, and a JavaScript testing best practices repository for testing; TypeORM and Mongoose documentation for databases; and Passport.js strategy docs plus RFC 8725 (JWT Best Current Practices) for authentication.

Nest.js developers debugging real dependency-injection, circular-dependency, database-connection, or authentication failures, or making architecture decisions about module organization, ORM choice, testing strategy, or caching, who want solutions grounded in actual reported GitHub/Stack Overflow issues and checked against a concrete success checklist, rather than generic framework advice with no way to confirm the fix actually holds.