Dependency Injection Lifetimes

TL;DR

Angular's hierarchical injectors decide how long a service instance lives and how widely it's shared: root providers are app-wide singletons, module/feature providers are singletons per feature boundary, and component providers create a fresh instance per component. Choosing the right scope controls state sharing and isolation — component-level providers, for example, keep state from leaking between instances and make unit tests cleaner.

How it works

Common scopes

• Root providers: Singleton for the entire app.
• Module/feature providers: Singleton per feature boundary.
• Component providers: New instance per component instance.

Testing tips

• Provide mocks in TestBed for unit tests.
• Use component-level providers to isolate state.

Interview prompt

• When would you scope a service at the component level?