# Providers

Basically, almost everything may be considered as a provider – service, factory, interceptors, and so on. All of them can inject dependencies, meaning, they can create various relationships with each other. But in fact, a provider is nothing else than just a simple class annotated with an @Injectable() decorator.

In controllers chapter, we've seen how to build a Controller, handle a request and create a response. Controllers shall handle HTTP requests and delegate complex tasks to the providers.

The providers are plain javascript class and use one of these decorators on top of them. Here the list:

# Services

Let's start by creating a simple CalendarService provider.

import {Injectable} from "@tsed/di";
import {Calendar} from "../models/Calendar";

@Injectable()
export class CalendarsService {
  private readonly calendars: Calendar[] = [];

  create(calendar: Calendar) {
    this.calendars.push(calendar);
  }

  findAll(): Calendar[] {
    return this.calendars;
  }
}
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Note @@Service@@ and @@Injectable@@ have the same effect. @@Injectable@@ accepts options, @@Service@@ does not.

A Service is always configured as singleton.

Example with @@Injectable@:

import {Injectable, ProviderScope, ProviderType} from "@tsed/di";
import {Calendar} from "../models/Calendar";

@Injectable({
  type: ProviderType.SERVICE,
  scope: ProviderScope.SINGLETON
})
export class CalendarsService {
  private readonly calendars: Calendar[] = [];

  create(calendar: Calendar) {
    this.calendars.push(calendar);
  }

  findAll(): Calendar[] {
    return this.calendars;
  }
}
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Now we have the service class already done, let's use it inside the CalendarCtrl:

import {BodyParams} from "@tsed/platform-params";
import {Get, Post} from "@tsed/schema";
import {Controller} from "@tsed/di";
import {Calendar} from "../models/Calendar";
import {CalendarsService} from "../services/CalendarsService";

@Controller("/calendars")
export class CalendarsController {
  constructor(private readonly calendarsService: CalendarsService) {
  }

  @Post()
  async create(@BodyParams() calendar: Calendar) {
    this.calendarsService.create(calendar);
  }

  @Get()
  async findAll(): Promise<Calendar[]> {
    return this.calendarsService.findAll();
  }
}
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Finally, we can load the injector and use it:

import {Configuration} from "@tsed/di";
import {CalendarsCtrl} from "./controllers/CalendarsCtrl";

@Configuration({
  mount: {
    "/rest": [CalendarsCtrl]
  }
})
export class Server {
}
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NOTE You'll notice that we only import the CalendarsCtrl and not the CalendarsService as that would be the case

with other DIs (Angular / inversify). Ts.ED will discover automatically services/providers as soon as it is imported into your application via an import ES6.

In most case, if a service is used by a controller or another service which is used by a controller, it's not necessary to import it explicitly!

# Dependency injection

Ts.ED is built around the dependency injection pattern. TypeScript emits type metadata on the constructor which will be exploited by the InjectorService to resolve dependencies automatically.

import {Injectable} from "@tsed/di";

@Injectable()
class MyInjectable {
  constructor(private calendarsService: CalendarsService) {
  }
}
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It's also possible to inject a service on a property by using Inject decorator:

import {Injectable, Inject} from "@tsed/di";

@Injectable()
class MyInjectable {
  @Inject()
  private calendarsService: CalendarService;

  $onInit() {
    console.log(this.calendarsService);
  }
}
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In this case, the service won't be usable in the constructor. If you have to do something with the injected service, you can use the $onInit hook.

# Scopes

All providers have a lifetime strictly dependent on the application lifecycle. Once the server is created, all providers have to be instantiated. Similarly, when the application shuts down, all providers will be destroyed. However, there are ways to make your provider lifetime request-scoped as well. You can read more about these techniques here.

# Binding configuration

All configurations set with Module or Configuration can be retrieved with Constant and Value decorators. These decorators can be used with:

Constant and Value accept an expression as parameter to inspect the configuration object and return the value.

import {Constant, Value} from "@tsed/di";
import {Env} from "@tsed/core";

export class MyClass {
  @Constant("env")
  env: Env;

  @Value("swagger.path")
  swaggerPath: string;

  $onInit() {
    console.log(this.env);
  }
}
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@@Constant@@ returns an Object.freeze() value.

NOTE The values for the decorated properties aren't available on constructor. Use $onInit() hook to use the

value.

# Custom providers

The Ts.ED IoC resolves relationships providers for you, but sometimes, you want to tell to the DI how you want to instantiate a specific service or inject different kind of providers based on values, on asynchronous or synchronous factory or on external library. Look here to find more examples.

# Configurable provider

Sometimes you need to inject a provider with a specific configuration to another one.

This is possible with the combination of Opts and UseOpts decorators.

import {Injectable, Opts, UseOpts} from "@tsed/di";

@Injectable()
class MyConfigurableService {
  source: string;

  constructor(@Opts options: any = {}) {
    console.log("Hello ", options.source); // log: Hello Service1 then Hello Service2

    this.source = options.source;
  }
}

@Injectable()
class MyService1 {
  constructor(@UseOpts({source: "Service1"}) service: MyConfigurableService) {
    console.log(service.source); // log: Service1
  }
}

@Injectable()
class MyService2 {
  constructor(@UseOpts({source: "Service2"}) service: MyConfigurableService) {
    console.log(service.source); // log: Service2
  }
}
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Using @@Opts@@ decorator on a constructor parameter changes the scope of the provider

to ProviderScope.INSTANCE.

# Lazy load provider 6.81.0+

By default, modules are eagerly loaded, which means that as soon as the application loads, so do all the modules, whether or not they are immediately necessary. While this is fine for most applications, it may become a bottleneck for apps running in the serverless environment, where the startup latency ("cold start") is crucial.

Lazy loading can help decrease bootstrap time by loading only modules required by the specific serverless function invocation. In addition, you could also load other modules asynchronously once the serverless function is "warm" to speed-up the bootstrap time for subsequent calls even further (deferred modules registration).

You can read more about these techniques here.

# Override provider

Any provider (Provider, Service, Controller, Middleware, etc...) already registered by Ts.ED or third-party can be overridden by your own class.

import {OriginalService} from "@tsed/common";
import {OverrideProvider} from "@tsed/di";

@OverrideProvider(OriginalService)
export class CustomMiddleware extends OriginalService {
  public method() {
    // Do something
    return super.method();
  }
}
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Just don't forgot to import your provider in your project !

Last Updated: 11/29/2021, 11:14:40 AM

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