Ugly TypeScript Monoid

ugly-typescript-monoid.ts

class MyNumber extends Number implements Monoid<MyNumber> {
    public x: Number;
    constructor(x: Number) {
        super(x)
        this.x = x;
    }

    toString(): string {
        return this.x.toString()
    }

    mappend(myNumber: MyNumber): MyNumber {
        return new MyNumber(this.x + myNumber.x)
    }

    mempty(): MyNumber {
        return new MyNumber(0)
    }
}

console.log('MyNumber append:', new MyNumber(5).mappend(new MyNumber(4))) // 9
console.log('MyNumber identity:', new MyNumber(5).mempty()) // 0