roman-to-arabic.js

function mapRomanToArabic (roman) {
  switch(roman) {
    case 'I':
      return 1
    case 'V':
      return 5
    case 'X':
      return 10
    case 'L':
      return 50
    case 'C':
      return 100
    case 'D':
      return 500
    case 'M':
      return 1000
    default:
      return 0
  }
}

function romanToArabic(str) {
  let romanArr = str.split('')
  let answer = 0

  for (let i = 0; i < romanArr.length; i++) {
    let curNum = mapRomanToArabic(romanArr[i])
    let nextNum = mapRomanToArabic(romanArr[i + 1])
    if (curNum >= nextNum) {
      answer += curNum
    } else {
      answer -= curNum
    }
  }
  
  return answer
}


/*
  I = 1
  V = 5
  X = 10
  L = 50
  
  http://www.mathatube.com/images/roman-numerals.JPG
*/

function testCases() {
  var tests = {
    'I': 1, // 0, 1 =>
    'IV': 4, // 0, 1, 5 => 
    'IX': 9, // 0, 1, 10 =>
    'VII': 7, // 0, 5, 1, 1 =>
    'XIX': 19, // 0, 10, 1, 10 =>
    'XXIV': 24, // 0, 10, 10, 1, 5 =>
    'XLIV': 44, // 0, 10, 50, 1, 5 =>
    'MMMCMXCIX': 3999
  };

  var keys = Object.keys(tests);

  keys.forEach(function (k) {
    var res = romanToArabic(k);

    if (res === tests[k]) {
      console.log(k + ' : Pass');
    } else {
      console.log(k + ' : Fail. Expected ' + tests[k] + ' but got ' + res );
    }
  });

}

testCases();

//