JSFiddle - React, Tailwind, and code Playground

JavaScript

n = {
    I: 1,
    V: 5,
    X: 10,
    L: 50,
    C: 100,
    D: 500,
    M: 1e3,
    IV: 4,
    IX: 9,
    XL: 40,
    XC: 90,
    CD: 400,
    CM: 900
}, s = prompt(), a = i = 0;
for (; i < s.length; i++) {
    b = (j) => s.substr(j, 1);
    c = b(i);
    d = b(i + 1);
    e = n[c + d];
    if (e) {
        a += e;
        i++;
    } else {
        a += n[c];
    }
}
alert(a);