Services In Angular2

Service in Angular2 is just a javascript function, along with the properties and methods that can be included in Angular2 component via dependancy injection. It allows us to write code for a specific task that can be used in components.

There are two methods or ways to create angular2 service file. one is to create manually and other is using angular command line interface.

There are several steps to create the theservive file.
Step 1. Create the Service File:- In the code editor, create a file with the name convention
Syntax: name.service.ts
In this syntax, "name" can be any name which is relevant to our service."service.ts" remains the same. we can put this file into app folder or in separate service folder.
Step 2. Import the Injectable Member:-
At the top of service file, we used toaddfollowing file
import { Injectable } from '@angular/core';
Step 3. Add Injectable Decorator:-
In this pace, we need to import the injectable member from a library, after that injectable() decorator function is added.

Syntax:
@Injectable()
Step4. Export the Service Class:-
And finally, this export class AuthenticationService {
    // This is where main methods and properties works, for example: 
    someMethod() {
        return 'Hello!';
    }
}

Second Way 
Creating the Service with the Angular-CLI
ng generateservicemyservicename
Importing the Service to Component
we can import the service directlyin the components or by using app.module.ts file. which gives access to the component of service file.
1. Step the service in the App component.

import { Component } from '@angular/core';
import { ExampleService } from './example.service';

2. Add it as a Provider

@Component({
    selector: 'my-app',
    template: '<h1>{{ title }}</h1>',
    providers: [ExampleService]
})

3. Include it using Dependency Injection
In the constructorargumentclass, we include it in the component using dependancy injection.
constructor(private _authenticationService: AuthenticationService) {
}
4. Using the service
we can access the service methods and properties by referencing the private_authenticationService

ngOnInit() {
        this.title = this._authenticationService.someMethod();
}

here is a full code of service file
authentication.service.ts

import { Injectable } from '@angular/core';
@Injectable()
export class AuthenticationService{
    someMethod() {
        return 'Hey!';
    }
}

app.component.ts

import { Component } from '@angular/core';
import { ExampleService } from './example.service';
@Component({
    selector: 'my-app',
    template: '<h1>{{ title }}</h1>',
    providers: [ExampleService]
})
export class AppComponent { 
    title: string;
    constructor(private _exampleService: ExampleService) {
    }
    ngOnInit() {
        this.title = this._exampleService.someMethod();
    }
}

Including the service in app.module.ts
// other imports in app.module.ts

import { AuthenticationService} from './authentcation.service';
@NgModule({
  //other metadata properties
  providers: [AuthenticationService]
})