typescript-open-closed-principle.ts

interface LanguageInterface {
    sayHello(): string;
}

class Persian implements LanguageInterface {
    public sayHello(): string {
        return 'درود';
    }
}

class French implements LanguageInterface {
    public sayHello(): string {
        return 'Bonjour';
    }
}

class Hello {
    public say(lang: LanguageInterface): string {
        return lang.sayHello();
    }
}

let obj = new Hello;
console.log(obj.say(new Persian));