Roman Numerals Converter

converter.js

//This script converts Roman numerals to decimal between I(1) to MMMCMXCIX(3999)

var numeral = "romanNumeral";

convertFromRoman(numeral);

function convertFromRoman(numeral) {
    var numbers = [];
    var previousNumber, currentNumber, sum, equalityTime, repeatedTimeOfV, repeatedTimeOfL, repeatedTimeOfD;
    sum = repeatedTimeOfV = repeatedTimeOfL = repeatedTimeOfD = 0;
    equalityTime = 2;
    var characters = numeral.split('');

    for (var i = 0; i < characters.length; i++) {
        if (characters[i] != "I" && characters[i] != "V" && characters[i] != "X" && characters[i] != "L" && characters[i] != "C" && characters[i] != "D" && characters[i] != "M") {
            alert("This is not a Roman numeral!");
            return;
        }

        switch (characters[i]) {
            case "I":
                currentNumber = 1;
                break;
            case "V":
                currentNumber = 5;
                repeatedTimeOfV++;
                break;
            case "X":
                currentNumber = 10;
                break;
            case "L":
                currentNumber = 50;
                repeatedTimeOfL++;
                break;
            case "C":
                currentNumber = 100;
                break;
            case "D":
                currentNumber = 500;
                repeatedTimeOfD++;
                break;
            case "M":
                currentNumber = 1000;
        }

        //* The symbols V, L and D are never repeated
        if (repeatedTimeOfV == 2 || repeatedTimeOfL == 2 || repeatedTimeOfD == 2) {
            alert("This is not a valid Roman numeral!");
            return;
        }

        if (i != 0) {
            if (previousNumber < currentNumber) {
                //The symbols V, L and D are never subtracted
                if (previousNumber == 5 || previousNumber == 50 || previousNumber == 500) {
                    alert("This is not a valid Roman numeral!");
                    return;
                }

                //The symbol I can be subtracted from V and X only
                if (previousNumber == 1 && (!(currentNumber == 5 || currentNumber == 10))) {
                    alert("This is not a valid Roman numeral!");
                    return;
                }

                //The symbol X can be subtracted from L, M, and C only
                if (previousNumber == 10 && (!(currentNumber == 50 || currentNumber == 100 || currentNumber == 1000))) {
                    alert("This is not a valid Roman numeral!");
                    return;
                }

                //A symbol cannot be subtracted more than once from a greater one
                if (i >= 2) {
                    if (previousNumber == numbers[i - 2]) {
                        alert("This is not a valid Roman numeral!");
                        return;
                    }
                }

                //A lower symbol is written to the left of greater one, it is subtracted from the greater one
                numbers[i - 1] = (-1 * previousNumber);
                equalityTime = 1;
            }
            else if (previousNumber == currentNumber) {
                if (i >= 2) {
                    if (previousNumber == numbers[i - 2])
                        equalityTime++;
                }
            }
            else { //previousNumber > currentNumber
                equalityTime = 1;
            }
        }

        numbers[i] = currentNumber;

        //A symbol is not repeated more than three times
        if (equalityTime > 3) {
            alert("This is not a valid Roman numeral!");
            return;
        }

        previousNumber = currentNumber;
    }

    numbers.forEach(function (item) {
        sum = sum + item;
    })

    alert(sum);
}