Maybe monad in TypeScript

maybe.ts

export class Maybe<A> {
    readonly kind: "Just" | "Nothing";
    value?: A;

    private constructor(kind: "Just" | "Nothing", value?: A) {
        this.kind = kind;
        this.value = value;
    }

    static readonly just = <A>(value: A): Maybe<A> => new Maybe("Just", value);

    static readonly nothing = <A>(): Maybe<A> => new Maybe("Nothing");

    get(): A {
        if (this.isNothing()) {
            throw new Error(`Cannot retreive value from Nothing`);
        }

        return this.value as A;
    }

    getOrDefault(fallback: A): A {
        return this.isNothing() ? fallback : (this.value as A);
    }

    getOrElse(fn: () => A): A {
        return this.isNothing() ? fn() : (this.value as A);
    }

    isNothing() {
        return this.kind === "Nothing";
    }

    map<A, B>(f: (a: A) => B): Maybe<B> {
        return this.isNothing()
            ? Maybe.nothing()
            : Maybe.just(f(this.value as A));
    }

    flatMap<A, B>(f: (a: A) => Maybe<B>): Maybe<B> {
        return this.isNothing() ? Maybe.nothing() : f(this.value as A);
    }
}

export const just = Maybe.just;
export const nothing = Maybe.nothing;