Dependency injection (DI) ni muundo ambapo kitu kinachopokea dependencies kutoka nje (injected) badala ya kuzizalisha yenyewe. Inakuza loose coupling, testability, na flexibility — muundo wa msingi, unaotumiwa sana katika programu za kisasa.
Dependency injection ni nini
WITHOUT DI → a class CREATES its own dependencies (tightly coupled):
class OrderService { constructor() { this.db = new Database(); } } // hardcoded dependency
WITH DI → dependencies are PROVIDED (injected) from outside:
class OrderService { constructor(db) { this.db = db; } } // db is injected
→ the object doesn't create/control its dependencies → they're given to it
