NestJS

The mental model of every Nest application: modules, controllers, providers and dependency injection, then each stage of the pipeline Nest runs around a request handler. One lesson per page of the official docs' Overview chapter, on one growing cats API.

The dependency injection container underneath every Nest application: what a provider token really is, factories, async and dynamic modules, scopes, cycles, the module reference, lazy loading, lifecycle hooks and metadata. One lesson per page of the official docs' Fundamentals chapter, on the same cats API.

Tests that run in your browser against real Nest classes: unit tests with the testing module, mocks and spies, controllers and guards in isolation, end-to-end tests over HTTP, and writing the code to make a given test pass. Every test you write is checked by bugs it has to catch.

The docs' Techniques chapter on the same cats API: configuration from .env, validation and serialization, a real database with TypeORM, then the HTTP toolbox (versioning, cookies, sessions, uploads, streamed files, server-sent events, calling other services) and what runs behind a request (caching, logging, events, scheduled tasks).

The docs' Security chapter on the same cats API: sign visitors in with JWTs and hash their passwords, decide what each of them may do with roles and policies, do the same the Passport way, then harden the service with helmet, CORS, CSRF protection and rate limiting, and finish with the whole API locked down.

Describe the cats API so that people and tools can read it: generate an OpenAPI document from the code you already have, shape every schema, parameter and response with the @nestjs/swagger decorators, declare how callers authenticate, reuse DTOs through mapped types, and serve more than one specification from the same app.