Existing Class Extending in Typescript

extendNumber.ts

// typing of implementation is not checked by the compiler, only calls to it
declare global {
    interface Number {
        between: (min: number, max: number) => boolean;
    }
}

Object.defineProperty(Number.prototype, 'between', {
    value(min: number, max: number): boolean {
        return this >= min && this <= max;
    },
});

const p = 5;
console.log(p.between(5, 10)); // true
console.log(p.between(0, 4)); // false