Angular Dependency Injection Explained: From Constructor Basics to Modern DI

Angular's Dependency Injection (DI) system has evolved well beyond simple constructor-based service injection, now offering tools like the inject() function, InjectionToken, hierarchical injectors, and tree-shakable providers. At its core, DI separates the responsibility of creating dependencies from the classes that use them, reducing tight coupling and improving testability. The system relies on three key concepts — tokens, providers, and injectors — to manage how objects are created, scoped, and supplied across an application. Modern Angular extends DI into an architectural pattern by supporting dependency inversion, allowing high-level modules to depend on abstractions rather than concrete implementations. Features like useClass, useFactory, functional guards, and environment-level providers make Angular DI a powerful tool for building scalable, maintainable applications.