Discussion on: Dependency injection in TypeScript applications powered by InversifyJS

[Remo H. Jansen]

Thanks! Good question, InversifyJS allows you to have more control over the life-cycle of dependencies and how the dependency graph is composed than a dictionary of singletons. InversifyJS gives you more power thanks to 3 main features:

Scopes

At the moment the core library allows:

• Transient scope (one instance each time you call container.get())
• Singleton scope (same instance for every call to container.get())

I'm working on custom scopes which will allow singletons within a context as opposed to application-level singletons. This will enable things like "Request scope" which can be used to declare singletons at HTTP-request-level.

Contextual constraints

You can inject dependencies based on execution time constraints. InversifyJs supports many types of constraints:

interface BindingWhenSyntax<T> {
    when(constraint: (request: interfaces.Request) => boolean): interfaces.BindingOnSyntax<T>;
    whenTargetNamed(name: string): interfaces.BindingOnSyntax<T>;
    whenTargetTagged(tag: string, value: any): interfaces.BindingOnSyntax<T>;
    whenInjectedInto(parent: (Function|string)): interfaces.BindingOnSyntax<T>;
    whenParentNamed(name: string): interfaces.BindingOnSyntax<T>;
    whenParentTagged(tag: string, value: any): interfaces.BindingOnSyntax<T>;
    whenAnyAncestorIs(ancestor: (Function|string)): interfaces.BindingOnSyntax<T>;
    whenNoAncestorIs(ancestor: (Function|string)): interfaces.BindingOnSyntax<T>;
    whenAnyAncestorNamed(name: string): interfaces.BindingOnSyntax<T>;
    whenAnyAncestorTagged(tag: string, value: any): interfaces.BindingOnSyntax<T>;
    whenNoAncestorNamed(name: string): interfaces.BindingOnSyntax<T>;
    whenNoAncestorTagged(tag: string, value: any): interfaces.BindingOnSyntax<T>;
    whenAnyAncestorMatches(constraint: (request: interfaces.Request) => boolean): interfaces.BindingOnSyntax<T>;
    whenNoAncestorMatches(constraint: (request: interfaces.Request) => boolean): interfaces.BindingOnSyntax<T>;
}

You can do something like:

container.bind<Weapon>("Weapon").to(Katana).whenInjectedInto(Samurai);
container.bind<Weapon>("Weapon").to(Shuriken).whenInjectedInto(Ninja);

Interception

InversifyJS allows you to implement interception thanks to the onActivation handlers and the @postConstruct decorator. You can learn more about IoC and interception here.

[Theofanis Despoudis]

Great. I suppose this fine level of control will suit more to backend applications or really complex web apps with lots of services when you need something more customizable.

[Remo H. Jansen]

Yes, InversifyJS becomes more valuable as your application grows.