Question 1

What is an HttpInterceptor?

Accepted Answer

Middleware that transforms requests/responses globally (auth headers, logging, errors). Functional interceptors with provideHttpClient(withInterceptors([...])) are modern. Order matters. Always clone requests when modifying.

Question 2

How do you attach an auth token with an interceptor?

Accepted Answer

Clone the request, set Authorization header from an auth service, then call next. Skip public endpoints. Handle 401 by refreshing tokens carefully to avoid infinite loops.

Question 3

How can interceptors cause infinite retry loops?

Accepted Answer

Refreshing tokens on 401 and retrying without guarding refresh failures, or re-entering the same interceptor path. Use a single-flight refresh, fail-fast on refresh error, and whitelist the refresh URL.

Question 4

When should logic be an interceptor vs a service method?

Accepted Answer

Cross-cutting concerns for all HTTP → interceptor; feature-specific shaping → service. Don't hide business rules in interceptors. Keep them thin and testable.

Interceptors Angular Interview Questions

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