JSFiddle - React, Tailwind, and code Playground

JavaScript

function t(i){for(p=l=n=0;a={I:1,V:5,X:10,L:50,C:100,D:500,M:1e3}[i[p++]];l=a)n+=a>l?-l:l;return n+l};

alert(t('MLII'));