Exemplary explanation of interfaces and generics

generics.ts

interface BarkAndWiggle {
    bark: () => void;
    wiggle: () => void;
};

class Pitbull implements BarkAndWiggle {
    public bark() {
        console.log('bark');
    }

    public wiggle() {
        console.log('happy');
    }
}

class Pug implements BarkAndWiggle {
    public bark() {
        console.log('yaff');
    }

    public wiggle() {
        console.log('tiny happy');
    }
}

const buck: Pitbull = new Pitbull();
const frank: Pug = new Pug();

function MakeBarkAndReturn<T extends BarkAndWiggle> (animal: T): T {
    animal.bark();

    return animal;
}


const someAnimal = MakeBarkAndReturn(frank); // someAnimas is of type 'Pug'

const foo: Array<string> = ['foo', 'bar'];
const pets = new Array<BarkAndWiggle>(frank, buck);


class Alien<T extends BarkAndWiggle> {
    private foodInStomach: T;

    public eat(food: T) {
        this.foodInStomach = food;
    }

    public pokeBelly(): T {
        this.foodInStomach.bark();

        return this.foodInStomach;
    }
} 

const zargon = new Alien<Pitbull>();

zargon.eat(frank); // ERROR wrong type of food
zargon.eat(buck); // Ok, edible

const result = zargon.pokeBelly(); // outputs 'bark', result is of type 'Pitbull'