Optional dependencies

We can declare an optional dependency using the @optional() decorator:

@injectable()
class Katana {
    public name: string;
    public constructor() {
        this.name = "Katana";
    }
}

@injectable()
class Shuriken {
    public name: string;
    public constructor() {
        this.name = "Shuriken";
    }
}

@injectable()
class Ninja {
    public name: string;
    public katana: Katana;
    public shuriken: Shuriken;
    public constructor(
        @inject("Katana") katana: Katana,
        @inject("Shuriken") @optional() shuriken: Shuriken // Optional!
    ) {
        this.name = "Ninja";
        this.katana = katana;
        this.shuriken = shuriken;
    }
}

let container = new Container();

container.bind<Katana>("Katana").to(Katana);
container.bind<Ninja>("Ninja").to(Ninja);

let ninja = container.get<Ninja>("Ninja");
expect(ninja.name).to.eql("Ninja");
expect(ninja.katana.name).to.eql("Katana");
expect(ninja.shuriken).to.eql(undefined);

container.bind<Shuriken>("Shuriken").to(Shuriken);

ninja = container.get<Ninja>("Ninja");
expect(ninja.name).to.eql("Ninja");
expect(ninja.katana.name).to.eql("Katana");
expect(ninja.shuriken.name).to.eql("Shuriken");

In the example we can see how the first time we resolve Ninja, its property shuriken is undefined because no bindings have been declared for Shuriken and the property is annotated with the @optional() decorator.

After declaring a binding for Shuriken:

container.bind<Shuriken>("Shuriken").to(Shuriken);

All the resolved instances of Ninja will contain an instance of Shuriken.

Default values

If a dependency is decorated with the @optional() decorator, we will be able to to declare a default value just like you can do in any other TypeScript application:

@injectable()
class Ninja {
    public name: string;
    public katana: Katana;
    public shuriken: Shuriken;
    public constructor(
        @inject("Katana") katana: Katana,
        @inject("Shuriken") @optional() shuriken: Shuriken = { name: "DefaultShuriken" } // Default value!
    ) {
        this.name = "Ninja";
        this.katana = katana;
        this.shuriken = shuriken;
    }
}