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okay, let's dive into a comprehensive guide on using class-based http interceptors in angular 18 (or the latest angular version, if available). we'll cover the core concepts, setup, implementation, and best practices, providing code examples throughout.

*what are http interceptors?*

http interceptors in angular provide a way to intercept and modify http requests and responses globally before they are sent to the server or delivered to the application. they act as intermediaries, allowing you to perform tasks like:

*adding headers:* appending authorization tokens (jwts), content types, or custom headers to all outgoing requests.
*transforming requests:* modifying request bodies, urls, or other request properties.
*handling errors:* catching and processing errors from the server (e.g., 401 unauthorized, 500 internal server error) to provide a consistent error handling mechanism.
*caching:* implementing client-side caching strategies.
*logging:* logging request and response details for debugging or auditing.
*showing loaders:* displaying loading indicators while requests are in progress.

*why use class-based interceptors?*

while you can technically achieve interception using functional interceptors, class-based interceptors offer better maintainability, testability, and structure, especially for complex interception logic. they allow you to leverage dependency injection, inheritance, and other object-oriented principles.

*key concepts*

*`httpinterceptor` interface:* the core interface that defines the structure of an interceptor. your interceptor classes must implement this interface.
*`httprequest`:* represents the outgoing http request. you can clone and modify `httprequest` objects.
*`httphandler`:* represents the next interceptor in the chain. you must call `next.handle(modifiedrequest)` to pass the request to the subsequent interceptor or the actual http backend.
*`httpevent`:* a union ...

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