Question 1

How does Angular dependency injection work?

Accepted Answer

Angular resolves constructor (or inject()) dependencies from hierarchical injectors. Providers register tokens → implementations. Components, routes, and root injectors form a hierarchy. Child injectors can override parents. inject() works in injection contexts.

Question 2

What is providedIn: 'root' vs providing in a component?

Accepted Answer

Root creates an app-wide singleton; component providers create an instance per component injector. Use root for shared services. Provide on a component/route to scope state to a feature subtree. Mis-scoping causes shared mutable state bugs.

Question 3

What are InjectionToken and multi providers?

Accepted Answer

InjectionToken identifies non-class dependencies; multi: true collects an array of providers. Use tokens for config objects and interfaces. HTTP_INTERCEPTORS is a classic multi provider. Prefer opaque tokens over string tokens.

Question 4

When do you use useClass, useValue, useFactory, and useExisting?

Accepted Answer

They map a token to a class, literal, factory function, or alias of another token. Factories enable runtime config. useExisting aliases tokens. Prefer useClass for simple swaps in tests.

Question 5

What is an injection context and why does inject() fail outside it?

Accepted Answer

inject() must run during construction/DI setup (or runInInjectionContext), not in arbitrary async callbacks. Call inject() in field initializers, constructors, or factories. Capturing Injector and running later is the escape hatch. This trips people migrating from constructor DI.

Question 6

How do you provide different implementations per environment?

Accepted Answer

Use environment providers, APP_INITIALIZER factories, or file replacements with alternate providers. Keep interfaces stable; swap implementations for mock/API. Avoid branching business logic on environment strings everywhere.

Question 7

Explain hierarchical injectors in a routed Angular app.

Accepted Answer

Root → platform/app → lazy route injectors → component injectors form a lookup chain. Lazy routes get their own injector for provided services. Element injectors (directives/components) participate too. Understanding the chain explains 'NullInjectorError' and unexpected singletons.

Dependency Injection Angular Interview Questions

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