Dependency injection (DI) er et mønster, hvor et objekt modtager sine dependencies udefra (injiceret) i stedet for at oprette dem selv. Det fremmer løs kobbling, testabilitet og fleksibilitet — et grundlæggende, bredt anvendt mønster i moderne software.
Hvad dependency injection er
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
