inject.ts

import 'reflect-metadata';

interface Type<T> {
  new(...args: any[]): T;
}

export const Injector = new class extends Map {
  resolve<T>(target: Type<T>): T {
    let tokens = Reflect.getMetadata('design:paramtypes', target) || [];

    console.log('Target', target.name);

    let injections = tokens.map((token) => Injector.resolve(token));

    let instance = this.get(target)
    if (instance) {
      return instance;
    }

    instance = new target(...injections);

    console.log(`Instanciating ${instance.constructor.name}`)
    this.set(target, instance);

    return instance;
  }
};

export function Injectable(): ClassDecorator {
  return function (target) { };
}