Understanding Injectable Decorator in Angular for HTTP Interceptors

In Angular, HTTP Interceptors are a unique and incredibly useful feature that allows developers to catch HTTP requests and responses for examination, transformation, or logging purposes. Since these interceptors potentially handle sensitive data and critical operations, their creation deserves special attention.

According to the illustrative quiz question, the correct decorator used to generate a new HTTP interceptor in Angular is @Injectable. While words like @HttpInterceptor or @Interceptor might seem more relevant, it’s actually @Injectable that does the job in this scenario.

The @Injectable Decorator

Angular's @Injectable decorator is what makes Dependency Injection (DI) possible in the Angular ecosystem. Decorators themselves are a special type of declaration in JavaScript & TypeScript. They can be attached to classes, methods, and properties, essentially changing or enhancing their behavior.

The @Injectable decorator indicates that a class, or more precisely a service, can be used with a DI mechanism. This is required for the HTTP Interceptors since these are services that should be provided in the root or any specific module to perform their duty.

Here is how to create a new HTTP Interceptor using the @Injectable decorator:

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler } from '@angular/common/http';

@Injectable()
export class CustomHttpInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest<any>, next: HttpHandler) {
    // Handle the request or response here
  }
}

In this case, the CustomHttpInterceptor service class is an HTTP Interceptor because it implements the HttpInterceptor interface. The @Injectable decorator marks it as injectable and ready for DI.

After creating this interceptor, it should be added to the HTTP_INTERCEPTORS array of the application's root module or any other module where you want it to act:

import { HTTP_INTERCEPTORS } from '@angular/common/http';
import { CustomHttpInterceptor } from './custom-http.interceptor';

@NgModule({
    ...
    providers: [
        { provide: HTTP_INTERCEPTORS, useClass: CustomHttpInterceptor, multi: true }
    ]
})

Final Thoughts

In conclusion, although the decorator @Injectable is widely used for various purposes in Angular, its application in creating HTTP Interceptors is a direct and critical one. It paves the way for these special services to be provided across the application, intercepting and potentially altering all HTTP communications for advanced functionality and robust handling. Best practices suggest that it should always be in the developers' toolset when working on any Angular project involving HTTP operations.