function add(a, b) { return a + b; } function subtract(a, b) { return a - b; } function sum(array) { return array.reduce((current, total) => total + current, 0); } function multiply(array) { return array.length ? array.reduce((accumulator, nextItem) => accumulator * nextItem) : 0; } function power(a, b) { return Math.pow(a, b); } function factorial(n) { if (n === 0) {return 1}; return n * factorial(n -1) } module.exports = { add, subtract, sum, multiply, power, factorial }