WTF is dependency injection

A dependency can be thought of as anything your code depends on in order to work.

As an example, say we have a class like below:

import { dependency } from 'dependency-library';

class ExampleClass = {

  public doSomething() {
    dependency.doSomething();
  }

}

We can say that ExampleClass depends on dependency-library, or dependency-library is a dependency of ExampleClass.

In this example we could say that ExampleClass is tightly coupled to dependency-library.

Any time we create a new ExampleClass(), we will also directly pull in dependency-library.

If for some reason we don’t want these two to be directly coupled, we could use dependency injection:

Dependency injection version:

class ExampleClass = {

  constructor(private dependency: DependencyLibrary) {}

  public doSomething() {
    this.dependency.doSomething();
  }

}

const instantiatedExampleClass = 
  new ExampleClass(new DependencyLibrary());

Why bother?

• Generally decoupled code is easier to modify and less fragile.
• It makes unit testing very simple as spying on injected dependencies and passing in simpler implementations is easy.
• It can allow for highly configurable code where different implementations that use the same public interface can be subbed in where needed.