Roman Numerals, in TypeScript

roman.test.ts

class Roman {
  constructor(private readonly str: string) {}

  private static LOOKUP = {
    I: 1,
    V: 5,
    X: 10,
    L: 50,
    C: 100,
    D: 500,
    M: 1000,
  };

  toNumber() {
    return this.str.split('').reduce((acc, _, i) => acc + this.valueOf(i), 0);
  }

  private valueOf(i: number) {
    return Roman.LOOKUP[this.str[i]] * this.factor(i);
  }

  factor(i: number) {
    return Roman.LOOKUP[this.str[i + 1]] > Roman.LOOKUP[this.str[i]] ? -1 : 1;
  }
}

const of = (str: string) => new Roman(str).toNumber();

test('I = 1', () => {
  expect(of('I')).toBe(1);
});
test('II = 2', () => {
  expect(of('II')).toBe(2);
});
test('III = 3', () => {
  expect(of('III')).toBe(3);
});
test('V = 5', () => {
  expect(of('V')).toBe(5);
});
test('IV = 4', () => {
  expect(of('IV')).toBe(4);
});
test('X = 10', () => {
  expect(of('X')).toBe(10);
});
test('IX = 9', () => {
  expect(of('IX')).toBe(9);
});
test('L = 50', () => {
  expect(of('L')).toBe(50);
});
test('XL = 40', () => {
  expect(of('XL')).toBe(40);
});
test('C = 100', () => {
  expect(of('C')).toBe(100);
});
test('XC = 90', () => {
  expect(of('XC')).toBe(90);
});
test('D = 500', () => {
  expect(of('D')).toBe(500);
});
test('M = 1000', () => {
  expect(of('M')).toBe(1000);
});

test('Various integrations', () => {
  expect(of('LXXXVIII')).toBe(88);
  expect(of('XCVIII')).toBe(98);
});