Apply decorators to third-party classes

Issue

I've got a DI container class which requires the classes to resolve to be decorated with an @Injectable() decorator, in order for the decorated class to emit metadata and for further configuration (pretty much like Angular DI works, https://angular.io/guide/dependency-injection).

In order to register third-party classes to my container I need to apply the @Injectable decorator to classes which I'm not able to modify. Think of something like a logger library installed in node_modules which I'd like to have in my container.

Is there a way to apply decorators to third-party classes? I'm thinking of something like:

import {Container, Injectable} from "./Container";
import {Logger} from "@vendor/logger";

const container = new Container();

container.register(Logger, { useClass: Logger }); // not possible, since Logger is not decorated
container.register(decorate(Logger, Injectable), { useClass: Logger }); // something like this, which would allow for decorating a class with a certain decorator

The only solution I can think of now is to create a decorated proxy class which extends from the real logger class. But proxying all third party classes seems like a lot of overhead.

Any other ideas?

Edit to give some more information:

The @Injectable decorator is required because non-decorated classes do not emit metadata (which would make it impossible for the container to read and resolve its dependencies). It's simply implemented by adding a metadata key to the decorated class with a key of __INJECTABLE and a value of true - whenever new services are registered in the container it checks if the __INJECTABLE flag is set and throws an error otherwise.

Here's an example of a proxy class:

import {Container, Injectable} from "./Container";
import {Logger as ParentLogger} from "@vendor/logger";

// Proxy
@Injectable()
class Logger extends ParentLogger {
}

const container = new Container();
container.register(Logger, { useClass: Logger });

This should work I guess, but would require to create proxies for pretty much every third party class.

Solution

Your decorate function can look something like this:

function Injectable() { return (_: any) => {} }

class ParentLogger { }

// Proxy
@Injectable()
class Logger extends ParentLogger {
}

function decorate<T extends { new(...args: any[]): any }>(sourceClass: T): T{
    @Injectable()
    class destinationClass extends sourceClass
    {
        constructor(...args: any[]) {
            super(...args);
        }
    }
    return destinationClass;
}

const DecoratedLogger = decorate(ParentLogger);

Playground here.

Answered By - Tiberiu Maran