The primitive used for the instantiation of the individual dependencies in our Angular applications via the DI mechanism of the framework is called the injector. The injector contains a set of providers that encapsulate the logic for the instantiation of registered dependencies associated with tokens. We can think of tokens as identifiers of the different providers registered within the injector.

Let's take a look at the following snippet, which is located at ch5/ts/injector-basics/injector.ts:

import 'reflect-metadata';
import {
  ReflectiveInjector,
  Inject,
  Injectable,
  OpaqueToken
} from '@angular/core';

const BUFFER_SIZE = new OpaqueToken('buffer-size');

class Buffer {
  constructor(@Inject(BUFFER_SIZE) private size: Number) {
    console.log(this.size);
  }
}

@Injectable()
class Socket {
  constructor(private buffer: Buffer) {}
}

let injector = ReflectiveInjector...