In the previous post, we have seen how to intercept the http request and response by extending Http class from @angular/http module.
To recall an http interceptor is

Interceptors are components that intercept calls and are used for logging, auditing, security and to implement authentication.

An Http interceptor is a middleware that can hijack http request/call before it reaches to the server and its response before it reaches the actual targeted function.

In Angular2 there was no cleaner way to implement interceptors, But Angular4 has an elegant and cleaner way to do this.  

What are we going to do?

We will create an interceptor class that will intercept the response object to check if the response status is 401 that is Unauthorized then the user will be redirected to the login page.

How?

To Create interceptor we will create a class HttpServiceInterceptor that will implement the HttpInterceptor interface from @angular/common/http module and register the same as providers in app.modules.ts. Following are simple steps to achieve the same

1) Create a class (HttpServiceInterceptor) and implement HttpInterceptor interface

@Injectable()
export class HttpServiceInterceptor implements HttpInterceptor { }

2) Provide definition to intercept method of HttpInterceptor interface

intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
}

And the full code

// http-service-interceptor.ts
import { Injectable, Inject } from "@angular/core";
import { HttpEvent, HttpInterceptor, HttpHandler, HttpRequest, HttpResponse, HttpErrorResponse } from "@angular/common/http";
import { DOCUMENT } from "@angular/common";
import { Observable } from "rxjs/Observable";
import "rxjs/add/operator/do";
@Injectable()
export class HttpServiceInterceptor implements HttpInterceptor {
    constructor( @Inject(DOCUMENT) private document: any) { }
    intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
        return next.handle(req).do(
            (event: HttpEvent<any>) => {
                if (event instanceof HttpResponse) {
                    return event;
                }
            },
            (err) => {
                if (err instanceof HttpErrorResponse) {
                    if (err.status === 401) {
                        this.document.location.href =
                        `${this.document.location.protocol}//${this.document.location.hostname}:${this.document.location.port}/login`;
                    }
                }
            });
    }
}

3) Import HttpClientModule and HTTP_INTERCEPTORS from @angular/http/common module

import { HttpClientModule,HTTP_INTERCEPTORS } from "@angular/common/http"; // to make http request
import { HttpServiceInterceptor } from "../interceptors/http-service-interceptor"; // response interceptor

4) Make an entry for HttpClientModule in imports array

imports: [
  HttpClientModule
]

5) Add provider for HTTP_INTERCEPTORS and use the class (HttpServiceInterceptor) that we created for interception

providers: [{
   provide: HTTP_INTERCEPTORS,
   useClass: HttpServiceInterceptor,
   multi: true
}],

6) Then use HttpClient instead of http as your http service

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

7) Inject in the constructor

constructor(private http: HttpClient) { }

That’s all folks!! Isn’t it easy? Oh yes..

Interceptors are components that intercept calls and are used for logging, auditing, security and to implement authentication.

An Http interceptor is a middleware that can hijack http request/call before it reaches to the server and its response before it reaches the actual targeted function.

One of the most famous usages is to implement authentication. Interceptors are very useful when one wants to append a header in each request or want to manipulate the response before it reaches the actual calling party. It can also be used to show and hide loader on the screen based on http call.

Another use is to return json object as response as we use the .map method in every request’s response to return res.json, so the response can be hijacked or intercepted to do the same for all the requests.

Angular2  has not implemented the concept of interceptor but this can be done by extending the HTTP service. There are few steps that you need to follow to make this working.

1) Extend Http Service to provide your own functionality

// http-interceptor.ts
import { Injectable } from'@angular/core';
import { Http, XHRBackend, ConnectionBackend, RequestOptions, RequestOptionsArgs, Response, Headers, Request } from '@angular/http';
import{ Observable } from'rxjs/Observable';
import 'rxjs/Rx';
@Injectable()
export class HttpInterceptor extends Http {
     constructor(backend: ConnectionBackend, defaultOptions: RequestOptions) {
         super(backend, defaultOptions);
     }
     /**
     * Error handler to handle is there is any error 
     * @param error 
     * @param caught
     * @returns {ErrorObservable}
     */
     private onCatch(error: any, caught: Observable<any>): Observable<any> {
         if (error.status === 500) {
             return Observable.throw(newError(error.status));
         }
         else if (error.status === 401) {
             return Observable.throw(newError(error.status));
         }
     }
      
      // interceptor method to intercept request and response
     get(url: string, options?: RequestOptionsArgs): Observable<any> {
         // options and url can be updated here to append new headers
          options.headers = {'Authorization' : 'token'};
         return super.get(url, options)
                     .catch(this.onCatch)
                     .map((res: Response) => {
                         // on sucess return res.json
                         return res.json();
                      }, (error: any) => {
                           console.log("On error :(");
                      })
                     .finally(() => {
                         console.log('After the requset has been completed :)');
                     });
     }
}

2) Add this extended service in app.module.ts

providers: [{
    provide: HttpInterceptor,
    useFactory: httpInterceptor,
    deps: [XHRBackend, RequestOptions]
}]

Where httpInterceptor is an exported function that returns the new instance of extended HttpInterceptor class.

// to return an instance of HttpInterceptor class
export function httpInterceptor (backend: XHRBackend, defaultOptions: RequestOptions) {
    return new HttpInterceptor(backend, defaultOptions);
}

Instead of creating this function one can directly use the function in provider section but this creates problem while bundling the project. The following error can be encountered

ERROR in Error encountered resolving symbol values statically. Function calls are not supported. Consider replacing the function or lambda with a reference
to an exported function (position 45:5 in the original .ts file), resolving symbol AppModule in D:/SK/Study/Material/MDCAngular/src/app/app.module.ts

Now to use this extended class inject the implementation of HttpInterceptor class like this where you want to inject http service

constructor (private http: HttpInterceptor) { }

Further Reading

http://array151.com/blog/http-interceptor-angular4/

References

https://medium.com/aviabird/http-interceptor-angular2-way-e57dc2842462