There are so many ways to convert Roman numerals

solution1.js

const convertValueToSymbol = (() => {
  const SYMBOLS = {
    1000: 'M',
    500: 'D',
    100: 'C',
    50: 'L',
    10: 'X',
    5: 'V',
    1: 'I'
  };

  return (value, placeValue) => {
    const symbol = SYMBOLS[placeValue];
    const higherSymbol = SYMBOLS[placeValue * 10];
    const midSymbol = SYMBOLS[(placeValue * 10) / 2];

    return (value === 9
      ? [symbol, higherSymbol]
      : value >= 5
        ? [midSymbol, convertValueToSymbol(value - 5, placeValue)]
        : value === 4
          ? [symbol, midSymbol]
          : [symbol.repeat(value)]
    ).join('');
  };
})();

export const solution = number =>
  Array
    // Create an array of the individual digits in the provided
    // number so that we can work on them one-by-one.
    .from(number.toString())
    // For each digit, also calculate its place value in the original
    // number and return them as a 2-tuple.
    .map((item, index, array) =>
      // The exponent is calculated by determining our current
      // position in the array compared to how many elements are in
      // it. Note that in the case of the last element this
      // evaluates to 0, making the whole exponential expression
      // equal to 1 (R^0 = 1 axiomatically)
      [Number(item), Math.pow(10, array.length - index - 1)]
    )
    .map(tuple => convertValueToSymbol(...tuple))
    // Now concatenate all symbols into a single string.
    .join('');

solution2.js

export const solution2 = (() => {
  // Just a shortened notation that evaluates to an array of arrays
  // containing the denomination values and their symbol.
  const DENOMINATIONS = `1000:M 900:CM 500:D 400:CD 100:C 90:XC 50:L 40:XL 10:X 9:IX 5:V 4:IV 1:I`
    .split(' ')
    .map(pair => pair.split(':'));

  return number => {
    let value = number;
    const symbols = [];

    for (const [symbolValue, symbol] of DENOMINATIONS) {
      const count = Math.floor(value / Number(symbolValue));
      value -= Number(symbolValue) * count;
      symbols.push(symbol.repeat(count));
    }

    return symbols.join('');
  };
})();