user.ts

interface User { 
    fullName(): string;
    adult(): boolean;
}

class Citizen implements User { 
    constructor(private firstName: string, private lastName: string, private age: number) { 
    }

    fullName() { 
        return `${this.firstName} ${this.lastName}`;
    }

    adult() { 
        return this.age >= 18;
    }
}

const joe = new Citizen("Joe", "Doe", 18)

console.log(joe.fullName())
console.log(joe.adult());