NestJS Core Architecture: Modules, Controllers, Providers and Dependency Injection

NestJS is a Node.js framework that uses a modular architecture to build scalable, maintainable backend applications. Its code is organized into Modules, Controllers and Providers, with Dependency Injection handling dependency creation so developers don't instantiate objects manually. Modules, decorated with @Module(), group related functionality such as users, posts or authentication, and each declares its controllers, providers, imports and exports. Controllers handle incoming HTTP requests through decorator-defined routes and delegate work to services, which hold the business logic and are injected into other components. Separating controllers from services improves reusability, maintainability and unit testing.