#15: 🔋 Creating a Service

In Angular, a service is (typically) a JavaScript class that's responsible for performing a specific task needed by your application. In our todo-list application, we'll create a service that will be responsible for saving and managing all the tasks, and we'll use it by injecting it into the components.

Create a service with the Angular CLI:

ng g s services/todo-list

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Create the services folder inside the app folder. Then use the Angular Generator to create the service named todo-list.

This command will generate the service in the file src/app/services/todo-list.service.ts. The service is a simple Class called TodoListService. It has the decorator @Injectable which allows it to use Dependency Injection.

src/app/services/todo-list.service.ts

import { Injectable } from '@angular/core';

@Injectable({
  providedIn: 'root'
})
export class TodoListService {

  constructor() { }
}

Share data

Now we can move the todoList array from ListManagerComponent to our new service. Go to the generated service file, src/app/services/todo-list.service.ts, and add this code inside the TodoListService class just above the constructor:

src/app/services/todo-list.service.ts

private todoList: TodoItem[] = [
  {title: 'install NodeJS'},
  {title: 'install Angular CLI'},
  {title: 'create new app'},
  {title: 'serve app'},
  {title: 'develop app'},
  {title: 'deploy app'},
];

Make sure that the TodoItem interface is imported:

src/app/services/todo-list.service.ts

import { TodoItem } from '../interfaces/todo-item';

Create a method to return the list

We'll add a getTodoList method that will return the todoList array. The service will look like this:

src/app/services/todo-list.service.ts

import { Injectable } from '@angular/core';
import { TodoItem } from '../interfaces/todo-item';

@Injectable({
  providedIn: 'root'
})
export class TodoListService {

  private todoList: TodoItem[] = [
    {title: 'install NodeJS'},
    {title: 'install Angular CLI'},
    {title: 'create new app'},
    {title: 'serve app'},
    {title: 'develop app'},
    {title: 'deploy app'},
  ];

  constructor() { }

  getTodoList(): TodoItem[] {
    return this.todoList;
  }
}

Inject and use the service

After creating the service, we can inject it into our list-manager component. In Angular Dependency Injection is very simple. We pass it as a parameter in the constructor - the parameter's type is the class name of the service. Angular assigns the instance it created to the parameter name, and we can use it from within the constructor. Before implementing it ourselves, let's see how it works. This is the constructor within the ListManagerComponent class:

src/app/list-manager/list-manager.component.ts

constructor(todoListService: TodoListService) {
  todoListService.getTodoList();
}

Typescript helps us furthermore by giving a shortcut for assigning the parameter to a class member. By adding private or public before the parameter name it is automatically assigned to this. So instead of declaring and assigning the property by ourselves:

export class ListManagerComponent {
  todoListService: TodoListService;

  constructor(todoListService: TodoListService) { 
    this.todoListService = todoListService;
  }
}

...we can reduce a lot of code like this:

export class ListManagerComponent {

  constructor(private todoListService: TodoListService) { }
}

So let's go on and use the service in the list-manager component.

• Remove the hard-coded list from the component, keep only the todoList property declaration.

• Inject the TodoListService using the constructor.

export class ListManagerComponent {
  todoList: TodoItem[];

  constructor(private todoListService: TodoListService) { }

• Make sure the TodoListService is imported.

src/app/list-manager/list-manager.component.ts

import { TodoListService } from '../services/todo-list.service';

• Get the list from the service in the constructor.

src/app/list-manager/list-manager.component.ts

constructor(private todoListService: TodoListService) {
  this.todoList = this.todoListService.getTodoList();
}

You don't need to change anything in the template since we're assigning the list to the same property we used before. Seems like nothing has changed, but you can check that the list comes from the service by changing it from there (adding an item, changing a title, etc.).

If the list is not shown and no error occures in the compilation, the project might not be synced with the addition of the service file. Stop the running of ng serve in the terminal by clickin Ctrl+C and run it again.

💾 Save your code to GitHub

StackBlitz users - press Save in the toolbar and continue to the next section of the tutorial.

Commit all your changes by running this command in your project directory.

git add -A && git commit -m "Your Message"

Push your changes to GitHub by running this command in your project directory.

git push

See the results on StackBlitz