Singletons

Sometimes, you'll want a service to only be created once. This might be when it has side-effects, such as creating a web server.

TypeDI supports this use-case by allowing individual services to be marked as singletons. When they are, only one instance of them will ever be created over the lifetime of the application.

tip

Singletons are attached to the globally-available default container. This means that, regardless of the container which resolves them, the same value will always be returned.

Singletons are created very similarly to ordinary services, with one extra configuration parameter that must be declared.

As an example, let's create services for an application that hosts a web server. We'd only want that web server to be created once, so we'd mark it as a singleton.

src/webserver.service.ts

import { Service } from '@freshgum/typedi';
import http from 'http'; // Node's HTTP module

@Service(
  {
    singleton: true,
  },
  []
)
export class WebServerService {
  private server: http.Server;
  startServer() {
    this.server = http.createServer((request, response) => {
      response.write('Hello world!');
      response.end();
    });

    this.server.listen(3000);
  }
}

Once we've done that, we can then start the web server in our root service.

src/root.service.ts

import { Service } from '@freshgum/typedi';
import { WebServerService } from './webserver.service';

@Service([WebServerService])
export class RootService {
  constructor(private webServer: WebServerService) {}

  bootstrap() {
    this.webServer.startServer();
  }
}

Now, any services which use the web-server as a dependency, regardless of if they are child containers, will always refer to that same web server instance. This prevents multiple web servers from being started at once.

tip

Utilising singletons for run-once operations is broadly considered a good idea.

In the case of our demo application, if multiple web server instances were to start, you would experience runtime errors as only one server can be hosted on a port at a time.

On the contrary, TypeDI also supports creating multiple instances of a service.