How to intercept the response in angular 4 with HttpInterceptor

next.handle(req) returns an observable so you can subscribe to it:

return next.handle(req).map((event: HttpEvent<any>) => {
  if (event instanceof HttpResponse) {
    // do stuff with response and headers you want
    event.headers
    event.body
  }
  return event;
})

To learn more about mechanics behind interceptors read:

• Insider’s guide into interceptors and HttpClient mechanics in Angular

you also need to import library

    import 'rxjs/add/operator/map';

then you use as below. you also need to return the event object so that it can be received in your subscribe() function.

    return next.handle(req).map((event: HttpEvent<any>) => {
      if (event instanceof HttpResponse) {
        // do stuff with response and headers you want
      }
      return event; 
    });