Example function that will convert a roman numeral to a regular number

romanNumeralConverter.js

// roman numeral mapping
const NUMERALS = {
  I: 1,
  V: 5,
  X: 10,
  L: 50,
  C: 100,
  D: 500,
  M: 1000
};

/**
 * compares two values to rank them
 * @param {number} c1 - first value to compare
 * @param {number} c2 - second value to compare
 * @returns {number} - returns 0, 1 or -1 if the c1 is equal, greater or less than the c2
 */
const compare = (c1, c2) => {
  if (NUMERALS[c1] > NUMERALS[c2]) {
    return 1;
  } else if (NUMERALS[c1] === NUMERALS[c2]) {
    return 0;
  } else if (NUMERALS[c1] < NUMERALS[c2]) {
    return -1;
  }
};

/**
 * Given a Roman Numeral, convert it to a number
 * @param {string} romanNumeral - a valid Roman Numeral
 * @returns {number}
 */
const readRomanNumeral = romanNumeral => {
  let rnv = 0;
  const chars = [...romanNumeral];
  for (let i = 0; i < chars.length; ) {
    if (i + 1 <= chars.length) {
      const curChar = chars[i];
      const nextChar = chars[i + 1];
      const rank = compare(curChar, nextChar);
      switch (rank) {
        case -1:
          rnv += NUMERALS[nextChar] - NUMERALS[curChar];
          i += 2;
          break;
        default:
          rnv += NUMERALS[curChar];
          i++;
      }
    } else {
      rnv += NUMERALS[chars[i]];
      i++;
    }
  }
  return rnv;
};

// const tests = {
//   I: 1,
//   II: 2,
//   III: 3,
//   IV: 4,
//   VII: 7,
//   XI: 9,
//   XIII: 13,
//   XIV: 14,
//   XX: 20,
//   XLVII: 47,
//   MCD: 1400,
//   MCMXIV: 1914
// };

// Object.entries(tests).forEach(([k, v]) => {
//   console.log(`reading: ${k} value should be ${v} got: ${readRomanNumeral(k)}`);
// });

const tests = [
  {
    expected: 1,
    test: "I"
  },
  {
    expected: 5,
    test: "V"
  },
  {
    expected: 9,
    test: "IX"
  },
  {
    expected: 9,
    test: "VIIII"
  },
  {
    expected: 10,
    test: "X"
  },
  {
    expected: 14,
    test: "XIV"
  },
  {
    expected: 19,
    test: "XIX"
  },
  {
    expected: 50,
    test: "L"
  },
  {
    expected: 89,
    test: "LXXXIX"
  },
  {
    expected: 90,
    test: "XC"
  },
  {
    expected: 100,
    test: "C"
  },
  {
    expected: 499,
    test: "ID"
  },
  {
    expected: 500,
    test: "D"
  },
  {
    expected: 1000,
    test: "M"
  },
  {
    expected: 2,
    test: "II"
  },
  {
    expected: 7,
    test: "VII"
  },
  {
    expected: 9,
    test: "IX"
  },
  {
    expected: 20,
    test: "XX"
  },
  {
    expected: 505,
    test: "DV"
  },
  {
    expected: 919,
    test: "CMXIX"
  }
];

const results = tests.map(t => {
  const result = readRomanNumeral(t.test);
  return {
    passes: result === t.expected,
    expected: t.expected,
    result,
    test: t.test
  };
});

const testResults = results.filter(t => {
  return !t.passes;
});

results.forEach(tr => {
  console.log(tr);
});