frac.js

class Fraction {
   constructor(numerator, denominator) {
      if (typeof numerator !== 'number' || typeof denominator !== 'number') {
         throw 'Invalid Fraction (numerator and denominator must be numbers.';
      }
      if (denominator <= 0) {
         throw 'Invalid Fraction (divide by Zero)';
      }
      const [_num, _denom] = Fraction.simplify(numerator, denominator);
      this.numerator = _num;
      this.denominator = _denom;
   }

   static gcd(n, d) {
      return !d ? n : this.gcd(d, n % d);
   }

   static simplify(num, denom) {
      const gcd = this.gcd(num, denom);
      return [num / gcd, denom / gcd];
   }

   pretty() {
      return `${this.numerator}/${this.denominator}`;
   }

   toNumber() {
      return this.numerator / this.denominator;
   }

   safeOperand(_operand) {
      if (_operand instanceof Fraction) {
         return _operand;
      } else {
         const parsed = parseInt(_operand);
         if (Number.isNaN(parsed)) {
            throw 'Invalid operand.';
         } else {
            return new Fraction(parsed, 1);
         }
      }
   }

   add(_operand) {
      const { numerator: n, denominator: d } = this.safeOperand(_operand);
      this.numerator = this.numerator * d + n * this.denominator;
      this.denominator *= d;
      const [_num, _denom] = Fraction.simplify(
         this.numerator,
         this.denominator
      );
      this.numerator = _num;
      this.denominator = _denom;
      return this;
   }

   subtract(_operand) {
      const { numerator: n, denominator: d } = this.safeOperand(_operand);
      return this.add(new Fraction(-n, d));
   }

   multiply(_operand) {
      const { numerator: n, denominator: d } = this.safeOperand(_operand);
      this.numerator *= n;
      this.denominator *= d;
      const [_num, _denom] = Fraction.simplify(
         this.numerator,
         this.denominator
      );
      this.numerator = _num;
      this.denominator = _denom;
      return this;
   }

   divide(_operand) {
      const { numerator: n, denominator: d } = this.safeOperand(_operand);
      return this.multiply(new Fraction(d, n));
   }
}

const test = new Fraction(100, 50);
test
   .add(5)
   .subtract(new Fraction(40, 10))
   .divide('8');
console.log(test.pretty()); // 3/8