Horje

Angular 2 - Http Interceptor
Start
  Using our class instead of Angular's Http

After extending the Http class, we need to tell angular to use this class instead of Http class.

In order to do this, in our main module(or depending on the needs, just a particular module), we need to write in the providers section:

export function httpServiceFactory(xhrBackend: XHRBackend, requestOptions: RequestOptions, router: Router, appConfig: ApplicationConfiguration) {
  return  new HttpServiceLayer(xhrBackend, requestOptions, router, appConfig);
}

import { HttpModule, Http, Request, RequestOptionsArgs, Response, XHRBackend, RequestOptions, ConnectionBackend, Headers } from '@angular/http';
import { Router } from '@angular/router';

@NgModule({
  declarations: [ ... ],
  imports: [ ... ],
  exports: [ ... ],
  providers: [
    ApplicationConfiguration,
    {
      provide: Http,
      useFactory: httpServiceFactory,
      deps: [XHRBackend, RequestOptions, Router, ApplicationConfiguration]
} 
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

Note: ApplicationConfiguration is just a service I use to hold some values for the duration of the application
  Simple Class Extending angular's Http class 
import { Http, Request, RequestOptionsArgs, Response, RequestOptions, ConnectionBackend, Headers } from '@angular/http';
import { Router } from '@angular/router';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/empty';
import 'rxjs/add/observable/throw';
import 'rxjs/add/operator/catch';
import { ApplicationConfiguration } from '../application-configuration/application-configuration';

/**
* This class extends the Http class from angular and adds automaticaly the server URL(if in development mode) and 2 headers by default:
* Headers added: 'Content-Type' and 'X-AUTH-TOKEN'.
* 'Content-Type' can be set in any othe service, and if set, it will NOT be overwritten in this class any more.
*/
export class HttpServiceLayer extends Http {

  constructor(backend: ConnectionBackend, defaultOptions: RequestOptions, private _router: Router, private appConfig: ApplicationConfiguration) {
    super(backend, defaultOptions);
  }

  request(url: string | Request, options?: RequestOptionsArgs): Observable<Response> {
    this.getRequestOptionArgs(options);
    return this.intercept(super.request(this.appConfig.getServerAdress() + url, options));
  }

  /**
  * This method checks if there are any headers added and if not created the headers map and ads 'Content-Type' and 'X-AUTH-TOKEN'
  * 'Content-Type' is not overwritten if it is allready available in the headers map
  */
  getRequestOptionArgs(options?: RequestOptionsArgs): RequestOptionsArgs {
    if (options == null) {
      options = new RequestOptions();
    }
    if (options.headers == null) {
      options.headers = new Headers();
    }

    if (!options.headers.get('Content-Type')) {
      options.headers.append('Content-Type', 'application/json');
    }

    if (this.appConfig.getAuthToken() != null) {
      options.headers.append('X-AUTH-TOKEN', this.appConfig.getAuthToken());
    }

    return options;
  }

  /**
  * This method as the name sugests intercepts the request and checks if there are any errors.
  * If an error is present it will be checked what error there is and if it is a general one then it will be handled here, otherwise, will be
  * thrown up in the service layers
  */
  intercept(observable: Observable<Response>): Observable<Response> {

    //  return observable;
    return observable.catch((err, source) => {
      if (err.status == 401) {
        this._router.navigate(['/login']);
        //return observable;
        return Observable.empty();
      } else {
        //return observable;
        return Observable.throw(err);
      }
    });
  }
}
  Simple HttpClient AuthToken Interceptor (Angular 4.3+) 
import { Injectable } from '@angular/core';
import { HttpEvent, HttpHandler, HttpInterceptor, HttpRequest } from '@angular/common/http';
import { UserService } from '../services/user.service';
import { Observable } from 'rxjs/Observable';

@Injectable()
export class AuthHeaderInterceptor implements HttpInterceptor {

  constructor(private userService: UserService) {
  }

  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    if (this.userService.isAuthenticated()) {
      req = req.clone({
        setHeaders: {
          Authorization:  `Bearer ${this.userService.token}`
        }
      });
    }
    return next.handle(req);
  }
}

Providing Interceptor (some-module.module.ts)

{provide: HTTP_INTERCEPTORS, useClass: AuthHeaderInterceptor, multi: true},

Remarks

What we do with HttpServiceLayer class is extend the Http class from angular and add our own logic to it.

We then inject that class in the bootstrap class of the application and tell angular that were we import the Http class, in the back to insert the HttpServiceLayer.

Anywhere in the code we can simply import

import { Http } from '@angular/http';

But our class will be used for each call.

 

Read Article
https://develop.horje.com/learn/1434/reference

 

Horje