roman-numeral-to-decimal/index.js

A module that converts a string of Roman numerals into its decimal equivalent

'use strict';

// A dict that maps the Roman numeral chars to their corresponding decimal value
const romanNumeralMap = {
    'I': 1,
    'V': 5,
    'X': 10,
    'L': 50,
    'C': 100,
    'D': 500,
    'M': 1000
};

/**
 * This function converts a string representation of a Roman numeral into its 
 * decimal equivalent
 * @param {String} romanNums - A string representation of a Roman numeral
 * @returns {Number} - The decimal representation of the passed in Roman numeral
 *  will return -1 if the input is somehow invalid
 */
const romanNumeralToDecimal = (romanNums) => {
    
    // Variable holding the numerical total of the inputted string
    let total = 0;

    // Checking if the input is of expected type (String) and contains characters
    if (typeof romanNums !== 'string' || romanNums.length === 0) {
        return -1;
    }

    // Iterating through the String char by char converting each numeral to its
    // decimal equivalent with consideration for subtractive notation
    for (let i = 0; i < romanNums.length; i++) {
        // boolean checking there is a subsequent char and it is within the romanNumeralMap
        let nextCharIsValid = romanNums[i + 1] && romanNumeralMap[romanNums[i + 1]];

        if (nextCharIsValid && (romanNumeralMap[romanNums[i + 1]] > romanNumeralMap[romanNums[i]])) {
            total += romanNumeralMap[romanNums[i + 1]] - romanNumeralMap[romanNums[i]];
            i++;
        } else if (romanNumeralMap[romanNums[i]]) {
            total += romanNumeralMap[romanNums[i]];
        } else {
            return -1;
        }
    }

    return total;
}

module.exports = romanNumeralToDecimal;