RomanNumerals.js

// Coded and shared on Twitter by Florin Pop
// https://twitter.com/florinpop1705/status/1234951601842016256

// Create a RomanNumerals class that can convert
// a roman numeral to and from an integer value.
const rom = {
	M: 1000,
	CM: 900,
	D: 500,
	CD: 400,
	C: 100,
	XC: 90,
	L: 50,
	XL: 40,
	X: 10,
	IX: 9,
	V: 5,
	IV: 4,
	I: 1
};

const RomanNumerals = {
	toRoman(num) {
		let res = "";
		Object.keys(rom).forEach(key => {
			while (num >= rom[key]) {
				num -= rom[key];
				res += key;
			}
		});
		return res;
	},

	fromRoman(roman) {
		let res = 0;
		const arr = roman.split("");
		for (let i = 0; i < arr.length; i++) {
			let keyA = arr[i];
			let keyB = arr[i + 1];
			if (rom[keyA] < rom[keyB]) {
				res += rom[keyA + keyB];
				i++;
			} else {
				res += rom[keyA];
			}
		}
        return res;
	}
};