Angular 4.3 Interceptor not working

If you have already check that you are using the HttpClient service and you olny import HttpClientModule once but you still have problems check this:

If you are using providedIn: 'root' and your service is inside a module.

@Injectable({
  providedIn: 'root'
})
export class YourService {
    ...
}

Make sure that you haven't added the service in the providers array of your module.

@NgModule({
  ...
  providers: [
    YourService, // REMOVE THIS LINE
  ]
})
export class YourModule { }

That will overwrite your interceptors for that service.

TL;DR Make sure there is one import of the HttpClientModule in the entire app (recommened) or provide valid interceptor configuration for each one (ofc for testing only).

Make sure that HttpClientModule is not imported multiple times across different modules of the app. I had it imported in lazy loaded modules. Each import creates a new copy of HttpClient using configuration from the module where it is imported, so interceptors provided in a root module are overwritten.

The reason - you use old Http service instead of new service, introduced in Angular 4.3 - HttpClient (Http is going to be deprecated). Also, in the HttpClient JSON response type is assumed by default, so you should ommit .map(contacts => contacts.json()).

app.module.ts

...
import { HttpClientModule } from '@angular/common/http';

@NgModule({
 imports: [
   HttpClientModule,
   ...
 ],
 providers: [
   ...
   {provide: HTTP_INTERCEPTORS, useClass: TokenInterceptor, multi: true}
 ]
 ...
})

user.service.ts

...
import {HttpClient} from "@angular/common/http";

@Injectable()
export class UserService {

  constructor(public http: HttpClient) {
  }

  public getContacts(): Observable<any[]> {
    return this.http.get('/users/contacts');
  }
}