Dependency Injection in Node.js + TypeScript

Dependency injection (DI) is a design pattern that allows a class to receive its dependencies from external sources rather than creating them itself. This can be particularly useful in large or complex applications, as it helps to decouple the different parts of the system and make it easier to test and maintain.

TypeScript is a popular programming language that is a superset of JavaScript, and it supports DI out of the box. In this blog post, we will explore how to use DI in a TypeScript application.

There are a few different ways to implement DI in TypeScript. One common approach is to use constructor injection, where dependencies are passed to a class via its constructor function. For example:

class ServiceA {
  doSomething() {
    console.log('Doing something');
  }
}

class ServiceB {
  constructor(private serviceA: ServiceA) {}

  doSomethingElse() {
    this.serviceA.doSomething();
    console.log('Doing something else');
  }
}

const serviceA = new ServiceA();
const serviceB = new ServiceB(serviceA);
serviceB.doSomethingElse();

In this example, ServiceB depends on ServiceA, and it receives an instance of ServiceA via its constructor. This allows ServiceB to use the doSomething() method of ServiceA without having to know how to create an instance of ServiceA itself.

Another way to implement DI in TypeScript is to use an inversion of control (IoC) container. An IoC container is a tool that manages the creation and injection of…