Roman numbers parser const RomanNumber = require('roman-number'); // npm i -S caub/roman-number const num1 = RomanNumber('XX');const num2 = RomanNumber(40); console.log(num1.toInt(), num2.toString()) // 20 'XL' compatibility node.js >= 6 browser (Chrome, Firefox, Edge, IE11, Safari, ..) (requires String.prototype.repeat, that can be shimmed)