Check out example codes for "interceptor in angular 9". It will help you in understanding the concepts better.

Code Example 1

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

Code Example 2

// src/app/auth/auth.service.ts

import { Injectable } from '@angular/core';
import decode from 'jwt-decode';

@Injectable()
export class AuthService {  
  public getToken(): string {
    return localStorage.getItem('token');
  }  

	public isAuthenticated(): boolean {    
      // get the token
      const token = this.getToken();    
      // return a boolean reflecting 
      // whether or not the token is expired
      return tokenNotExpired(null, token);  
    }
}

// src/app/auth/token.interceptor.ts
import { Injectable } from '@angular/core';
import {
  HttpRequest,
  HttpHandler,
  HttpEvent,
  HttpInterceptor
} from '@angular/common/http';
import { AuthService } from './auth/auth.service';
import { Observable } from 'rxjs/Observable';
@Injectable()
export class TokenInterceptor implements HttpInterceptor {
  constructor(public auth: AuthService) {}
  intercept(request: HttpRequest<any>, next: HttpHandler): Observable<HttpEvent<any>> {
    
    request = request.clone({
      setHeaders: {
        Authorization: `Bearer ${this.auth.getToken()}`
      }
    });
    return next.handle(request);
  }
}

// src/app/app.module.ts
import { HTTP_INTERCEPTORS } from '@angular/common/http';
import { TokenInterceptor } from './../auth/token.interceptor';

@NgModule({  bootstrap: [AppComponent],
          	 imports: [...],
			 providers: [
                       { 
                       provide: HTTP_INTERCEPTORS,
                       useClass: TokenInterceptor,
                       multi: true   
                       }  
          ]})
export class AppModule {}

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