Quick and dirty rational numbers in TS

rational.ts

export type Rational = {
  numerator: number;
  denominator: number;
  simplify: () => Rational;
  add: (other: Rational) => Rational;
  sub: (other: Rational) => Rational;
  mul: (other: Rational) => Rational;
  div: (other: Rational) => Rational;
  valueOf: () => number;
  toString: () => string;
};

export const rational = (n: number, d = 1): Rational => ({
  numerator: n,
  denominator: d,

  simplify: () => {
    const gd = Math.abs(gcd(n, d));
    const g = d < 0 ? -gd : gd;
    return rational(n / g, d / g);
  },

  add: ({ numerator: n2, denominator: d2 }: Rational) =>
    rational(n * d2 + n2 * d, d * d2),

  sub: ({ numerator: n2, denominator: d2 }: Rational) =>
    rational(n, d).add(rational(-n2, d2)),

  mul: ({ numerator: n2, denominator: d2 }: Rational) =>
    rational(n * n2, d * d2),

  div: ({ numerator: n2, denominator: d2 }: Rational) =>
    rational(n, d).mul(rational(d2, n2)),

  valueOf: () => n / d,
  toString: () => `${n}/${d}`,
});

const gcd = (a: number, b: number): number => b === 0 ? a : gcd(b, a % b);