HKT in typescript don't work. But why?

hkt.ts

interface Functor<F, A> {
    map: <B>(this: Functor<F, A>, f: (a: A) => B) => Functor<F, B>
}

class Maybe<A> implements Functor<Maybe<A>, A> {
    private constructor(public value: A) { }
    static Nothing<A>() { return new Maybe<A>(undefined); }
    static Just<A>(value: A) { return new Maybe<A>(value); }

    map<B>(this: Maybe<A>, f: ((a: A) => B)): Maybe<B> {
        if (this.value === undefined) return Maybe.Nothing<B>();
        return Maybe.Just(f(this.value));
    }

    // // Strangely enough, this typechecks, too
    // map<B>(this: Maybe<A>, f: ((a: A) => B)): List<B> {
    //     if (this.value === undefined) return new List<B>([]);
    //     return new List([f(this.value)]);
    // }


    toString() {
        if (this.value === undefined) return 'undefined';
        return this.value.toString();
    }
}

class List<A> implements Functor<List<A>, A> {
    array: A[];

    constructor(as: A[]) {
        this.array = as;
    }

    map <B>(this: List<A>, f: ((a: A) => B)): List<B> {
        return new List<B>(this.array.map(f));
    }

    toString() {
        return this.array.toString();
    }
}

const l1 = new List(['aaa', 'aa', 'a']);
const j1 = Maybe.Just(20);