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An Overview On Angular Interceptor

In Angular application, the Http request and response can be inspected and transform using the Interceptors. An application can configure multiple interceptors if needed. Each interceptor gets invoked for every Http request.

Interceptor Flow:

The Interceptors moto is to intercepts and handles an 'HttpRequest' or 'HttpResponse'

Most interceptors transform the outgoing request before passing it to the next interceptor in the chain, by calling 'next.handle(request)'. An interceptor may transform the response event stream as well, by applying addition RXJS operators on the stream returned by 'next.handle()'

More rarely, an interceptor may handle the request entirely and compose a new event stream instead of invoking 'next.handle()'. This is acceptable behavior, but keep in mind that further interceptors will be skipped entirely.

It is also rare but valid for an interceptor to return multiple responses on the event stream for a single request.


  • Let's suppose consider we have 2 interceptors in our application. Once the application starts to call an API, our Http request enters the 1st interceptor there we will implement the most common logics like adding a custom header, adding access token, etc.
  • After that 'next.handle()'(next is HttpRequest Handler) will pass our Http request to the 2nd interceptor, then our Http request executes the logic in 2nd interceptor like the 1st one. 
  • In 2nd interceptor 'next.handle()' method executes since it is last interceptor request will be sent to the API. 
  • Now API response first will be captured by 2nd interceptor, here we can implement some common logics like filtering response or retry call for API if it fails, etc. after that Http response will enter to 1st interceptor will be processed similar way as in 2nd interceptor. 
  • Finally, the response will be captured by services  and the components

Create A Sample Angular App:

Let's understand interceptor features by implementing it in a sample angular application.

Command To Install Angular CLI:(If not installed on your system previously)

npm install -g @angular/cli
Angular CLI Command To Create Angular App:

ng new your_project_name
Command To Run App:

ng serve
Let's create a service that triggers an Http get request.
src/app/app.service.ts:
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';

@Injectable()
export class AppService {
  constructor(private httpClient: HttpClient) {}

  get(){
      this.httpClient.get("http://test.com/api")
      .subscribe(
          (response) => {console.log('success')},
          (error) => {console.log('fails')}
      )
  }
}
Here created a test angular service it helps to understand interceptors better way. The API used here is a dummy one.

Now inject the HttpClient and our service in the AppModule file.
src/app/app.module.ts:
import { HttpClientModule } from '@angular/common/http';

import { AppService } from './app.service';
// some code hidden for better display
@NgModule({
  imports: [HttpClientModule],
  providers: [AppService]
})
export class AppModule {}
  • (Line: 7) HttpClientModule imported which is available at '@angular/common/http'
  • (Line: 8) imported our 'AppService'
Now, let's add a button that triggers our dummy API.
src/app/app.component.ts:
import { Component } from '@angular/core';

import { AppService } from './app.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  title = 'demo-interceptor';

  constructor(private appSerive:AppService){

  }
  triggerService(){
    this.appSerive.get();
  }
}
  • Injected our 'AppService' into our component.
src/app/app.component.html: 
<div>
  <input type="button" value="Trigger Service" (click)="triggerService()">
</div>

Create A Test1Interceptor:

Let's create our first interceptor which is a dummy one does nothing with 2 console logs to track execution.
src/app/test1.interceptor.ts:
import { Injectable } from '@angular/core';
import {
  HttpInterceptor,
  HttpRequest,
  HttpHandler,
  HttpEvent,
} from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class Test1Interceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    console.log('here request can modify -- Interceptor 1')
    var handle = next.handle(req);
    console.log('here response can modify -- Interceptor 1')
    return handle;
  }
}
  • (Line: 11) HttpInterceptor is an interface that needs to be implemented by our interceptor class. The 'HttpInterceptor' interface loads from '@angular/common/http'
  • (Line: 12) 'intercept()' is an abstract method from 'HttpInterceptor' that we need to implement for identifying and handling an Http request. The 'req' parameter of type 'HttpRequest<any>' is for outgoing request object to handle. The 'next' parameter of type 'Http' is to trigger the next interceptor in the flow of the chain or the backend(API). If no interceptor remains in the chain, then returns an observable of the event stream.
  • (Line: 13&15) console log to understand interceptor execution.

Import Provider In AppModule:

To use the same instance of 'HttpInterceptor' for the entire application, import the 'HttpClientModule' only in our 'AppModule', and add the interceptors to the root application injector. If we import 'HttpClientModule' multiple times across different modules(for example, in lazy loading modules), each import creates a new copy of the 'HttpClientModule', which overwrites the interceptor provided in the root module.
src/app/app.module.ts:
import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http';
import { Test1Interceptor } from './test1.interceptor';

@NgModule({
  imports: [HttpClientModule],
  providers: [
    {
      provide: HTTP_INTERCEPTORS,
      useClass: Test1Interceptor,
      multi: true,
    }
  ],
  bootstrap: [AppComponent],
})
export class AppModule {}
  • (Line: 8) The 'HTTP_INTERCEPTORS' is a multi-provider token that represents the array of registered 'HttpInterceptor' objects.
  • (Line: 9) assign our interceptor class.
  • (Line: 10) 'multi' is required to set true, which allows us to have multiple interceptors in our application all will be injected with the help of multi-provider token 'HTTP_INTERCEPTORS'.

Test Interceptors:

Now track our console logs added in our interceptor to understand the flow.

For better understanding lets add one more interceptor as below.
src/app/test2.interceptor.ts:
import { Injectable } from '@angular/core';
import {
  HttpInterceptor,
  HttpRequest,
  HttpHandler,
  HttpEvent,
} from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class Test2Interceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    console.log('here request can modify -- interceptor 2')
    var handle = next.handle(req);
    console.log('here response can modify -- intereptor 2')
    return handle;
  }
}
Now inject the Test2Interceptor in AppModule like we did in previous steps.
src/app/app.module.ts:
import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http';

import { Test1Interceptor } from './test1.interceptor';
import { Test2Interceptor } from './test2.interceptor';


@NgModule({
   providers: [
    {
      provide: HTTP_INTERCEPTORS,
      useClass: Test1Interceptor,
      multi: true,
    },
    {
      provide: HTTP_INTERCEPTORS,
      useClass: Test2Interceptor,
      multi: true,
    }
  ]
})
export class AppModule {}
Now track the console log we can understand how the interceptors chain will execute in request and response.

Order Of Interceptor Execution:

The order of the interceptor's execution will be the same as they injected in the AppModule. So if we want to add or skip the interceptors dynamically then we need to add 'if' conditions inside of the interceptors either to execute or skip the transformation of Http request and Http response.

Transform Request Object:

Interceptors can be used to modify the HttpRequest object. It is not recommended to modify them directly. Even most of the HttpRequest object properties are read-only which means they immutable. But Interceptor meant for the transformation so instead of modifying the original Http request object here we will clone them and implement our transformation to the new request object. Using intercepted we can do things like URL modification(not recommended, only can be used in rare cases based on your application business), setting default headers, request caching, etc.

Let's add some custom headers in our sample interceptors for understanding.
src/app/test1.interceptor.ts:
intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
	const transformedReq = req.clone({
	  headers: req.headers.set('custom-header-1', 'im Interceptor 1')
	});
	return next.handle(transformedReq);
}
  • (Line: 2) 'req.clone()' method is used to set our custom header
  • (Line: 5) newly created HttpRequest object will be delivered to other interceptors if exist or sends to API.
src/app/test2.interceptors.ts: 
intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    const transformedReq = req.clone({
        headers: req.headers.set('custom-header-2', 'im Interceptor 2')
    });
    return next.handle(transformedReq);
}

Transform Response Object:

In interceptors, it is possible to modify the response object but it is not all recommended to modify the response.
intercept(req: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {

	return next.handle(req).pipe(
	  map((response:HttpResponse<any>) => {
		 return response.clone({statusText: "hurray"})
	  })
	);
}

Interceptor Providers index.ts:

An ideal way of exposing bulk number providers is by using a separate container file like index.ts. This approach makes AppModule configurations cleaner.
src/app/interceptors/index.ts:
import { HTTP_INTERCEPTORS } from '@angular/common/http';

import { Test1Interceptor } from '../test1.interceptor';
import { Test2Interceptor } from '../test2.interceptor';

export const interceptorProviders = [
  {
    provide: HTTP_INTERCEPTORS,
    useClass: Test1Interceptor,
    multi: true,
  },
  {
    provide: HTTP_INTERCEPTORS,
    useClass: Test2Interceptor,
    multi: true,
  },
];
Now import this 'interceptorProviders' array into the AppModule
src/app/app.module.ts:
import {interceptorProviders} from './interceptor/index';

@NgModule({
  providers: [
    interceptorProviders
  ]
})
export class AppModule {}

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Wrapping Up:

Hopefully, I think this article delivered some useful information on Angular Interceptors. I love to have your feedback, suggestions, and better techniques in the comment section below.

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