Now someone is going to define const isOdd = (i) => !isEven(i); and you’ll get trouble.

A proper solution would be along the lines of:

    const isRealNumber = (n) => {
      if (typeof n !== 'number') return false;
      if (isNaN(n)) return false;
      if (n === +Inf || n === -Inf) return false;
      return true;
    };

    const isEven = (n) => {
      if (!isNumber(n)) throw new Error("Numerical error: Invalid input");
      return n % 2 === 0;
    }

    const isOdd = (n) => {
      if (!isNumber(n)) throw new Error("Numerical error: Invalid input");
      return n % 2 !== 0;
    }

This is the additional complexity created by dynamically weakly typed languages. And that’s why we get all these BS tiny npm packages.