What are HTTP interceptors?

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Accepted Answer

**HTTP interceptors** sit in the middle of every `HttpClient` request/response, letting you inspect and transform them globally. They're the perfect place for **cross-cutting concerns** that apply to all requests — auth tokens, logging, error handling, loading indicators.

## A functional interceptor (modern Angular)

```ts
import { HttpInterceptorFn } from "@angular/common/http";
import { inject } from "@angular/core";

export const authInterceptor: HttpInterceptorFn = (req, next) => {
  const token = inject(AuthService).getToken();

// requests are immutable — CLONE to modify
  const authReq = req.clone({
    setHeaders: { Authorization: `Bearer ${token}` }, // attach token to EVERY request
  });

return next(authReq); // pass the (modified) request along the chain
};
```

The interceptor receives the request and a `next` function (the next handler). Crucially, requests are **immutable**, so you `.clone()` with changes rather than mutating. Registering this once means every outgoing request automatically carries the auth header — no repeating it in each service.

## Registering interceptors

```ts
bootstrapApplication(AppComponent, {
  providers: [
    provideHttpClient(withInterceptors([authInterceptor, errorInterceptor, loggingInterceptor])),
  ],
});
```

Multiple interceptors form a **chain**, each passing to the next — order matters.

## Handling responses and errors globally

```ts
import { catchError } from "rxjs/operators";
import { throwError } from "rxjs";

export const errorInterceptor: HttpInterceptorFn = (req, next) =>
  next(req).pipe(
    catchError((err) => {
      if (err.status === 401) inject(Router).navigate(["/login"]); // global 401 handling
      inject(NotificationService).showError(err.message);
      return throwError(() => err);
    })
  );
```

Since interceptors wrap the response Observable too, you can handle errors (e.g. redirect on 401, show a toast) in **one place** for the entire app.

## Common interceptor uses

```text
✓ Attach auth tokens to requests
✓ Global error handling (401 → login, show notifications)
✓ Logging / analytics of all HTTP traffic
✓ Show/hide a global loading spinner
✓ Add common headers, retry logic, caching
```

## Why it matters

Interceptors centralize logic that would otherwise be duplicated across every HTTP call — most importantly attaching auth tokens and handling errors globally.

This keeps services clean (they just make requests) and ensures consistent behavior app-wide.

The auth-token and global-error interceptors are near-universal in production Angular apps; knowing the clone-to-modify rule and the chain model is key to writing them correctly.