[Angular] Intercept HTTP requests in Angular

Being able to intercept HTTP requests is crucial in a real world application. Whether it is for error handling and logging or for injecting authentication tokens. While in Angular version 2 it intercepting HTTP requests was totally possible, implementing it wasn't that trivial and intuitive. Starting from Angular version 4.3.1 there is a new, way more simpler approach of implementing HTTP interceptors. In this lesson we're going to explore how.

http.intercept.ts:

import { Injectable } from '@angular/core';
import {
  HttpInterceptor,
  HttpRequest,
  HttpResponse,
  HttpErrorResponse,
  HttpHandler,
  HttpEvent
} from '@angular/common/http';

import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/do';
import 'rxjs/add/operator/catch';
import 'rxjs/add/observable/throw';

@Injectable()
export class MyHttpLogInterceptor implements HttpInterceptor {
  intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    console.log('processing request', request);

    const customReq = request.clone({
      headers: request.headers.set('app-language', 'it')
    });

    return next
      .handle(customReq)
      .do((ev: HttpEvent<any>) => {
        if (ev instanceof HttpResponse) {
          console.log('processing response', ev);
        }
      })
      .catch(response => {
        if (response instanceof HttpErrorResponse) {
          console.log('Processing http error', response);
        }

        return Observable.throw(response);
      });
  }
}

Register:

@NgModule({
  imports: [ BrowserModule, HttpClientModule ],
  providers: [
    { provide: HTTP_INTERCEPTORS, useClass: MyHttpLogInterceptor, multi: true }  
  ],
  declarations: [ App ],
  bootstrap: [ App ]
})
export class AppModule {}