13-romanToInteger.js

var romanToInt = function (s) {
    let arr = s.split("");
    let i = 0;
    let result = 0;

    while (arr[i] != undefined) {
        switch (arr[i]) {
            case "I":
                if (arr[i + 1] === "V") {
                    result += 4;
                    i++;
                } else if (arr[i + 1] === "X") {
                    result += 9;
                    i++;
                } else {
                    result += 1;
                }
                break;
            case "V":
                result += 5;
                break;
            case "X":
                if (arr[i + 1] === "L") {
                    result += 40;
                    i++;
                } else if (arr[i + 1] === "C") {
                    result += 90;
                    i++;
                } else {
                    result += 10;
                }
                break;
            case "L":
                result += 50;
                break;
            case "C":
                if (arr[i + 1] === "D") {
                    result += 400;
                    i++;
                } else if (arr[i + 1] === "M") {
                    result += 900;
                    i++;
                } else {
                    result += 100;
                }
                break;
            case "D":
                result += 500;
                break;
            case "M":
                result += 1000;
                break;
        }
        i++;
    }

    return result;
};

console.log(romanToInt("MCMXCIV"));