odin-javascript-exercises/calculator/calculator.js

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;
  let product = 1;
  for (let i = n; i > 0; i--) {
    product *= i;
  }
  return product;
}

// This is another implementation of Factorial that uses recursion
// THANKS to @ThirtyThreeB!
function recursiveFactorial(n) {
  if (n===0){
    return 1;
  }
  return n * recursiveFactorial (n-1);
}

module.exports = {
  add,
  subtract,
  sum,
  multiply,
  power,
  factorial
};