di_framework.md

DI Framework

So there's a fancy new Hamburger class that is easy to test, but it's currently awkward to work with. Instantiating a Hamburger requires:

const hamburger = new Hamburger(new Bun(), new Patty('beef'), new Toppings([]));

That's a lot of work to create a Hamburger, and now all the different pieces of code that make Hamburgers have to understand how Bun, Patty and Toppings get instantiated.

One approach to dealing with this new problem might be to make a factory function like so:

function makeHamburger() {
    const bun = new Bun();
    const patty = new Patty('beef');
    const toppings = new Toppings(['lettuce', 'tomato', 'pickles']);
    return new Hamburger(bun, patty, toppings);
}

This is an improvement, but when more complex Hamburgers need to be created this factory will become confusing. The factory is also responsible for knowing how to create four different components. This is a lot for one function.

This is where a dependency injection framework can help. DI Frameworks have the concept of an Injector object. An Injector is a lot like the factory function above, but more general, and powerful. Instead of one giant factory function, an Injector has a factory, or recipe (pun intended) for a collection of objects. With an Injector, creating a Hamburger could be as easy as:

const injector = new Injector([Hamburger, Bun, Patty, Toppings]);
const burger = injector.get(Hamburger);