As of Angular 6, we have access to a great way to have our services globally available in our applications, while still being bundled and lazy loaded as efficiently as possible. @Provider decorators now take an additional argument, providedIn.
In almost every case, you want to use providedIn: 'root' - telling your angular application that this service should be a singleton, available all throughout your application. You don't even need to inject it into a module!
app/greeter.service.ts
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import{ Injectable }from"@angular/core";
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​
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@Injectable({
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providedIn:"root"
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})
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exportclassGreeterService{
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private message:string=
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"Registering Providers while Bootstrapping an Angular 2 application!";