IntroduceStaticSetter.ts

export class MessageRouter {
    public route(message: Message): void {
        //!! ouch... x(
        ExternalRouter.getInstance().sendMessage(message);
    }
}

export class ExternalRouter { // another Singleton! x(
    private static instance: ExternalRouter | null = null;

    private constructor() {
        // initialize stuff
    }

    public static getInstance(): ExternalRouter {
        if (this.instance === null) {
            this.instance = new ExternalRouter();
        }
        return this.instance;
    }

    // more code...
    public sendMessage(message: Message): void {
        // interesting code to send the message
    }
}