ratioCalculator.js

const intersection = (arr1, arr2) => arr1.length > arr2.length 
  ? arr1.filter(x => arr2.includes(x))
  : arr2.filter(x => arr1.includes(x))

const getFactors = (number) => Array
  .from(Array(number), (_, i) => i)
  .filter(i => number % i === 0)

const calculateRatio = ([width, height]) => {
  const gcf = Math.max(...intersection(getFactors(width),getFactors(height)))
  return [width/gcf,height/gcf]
}

const calculateRatioHeight = ([width, height]) => (height/width)*100;