Here is my shortest solution:
function convertToRoman(num) {
let lookupTable = {M:1000,CM:900,D:500,CD:400,C:100,XC:90,L:50,XL:40,X:10,IX:9,V:5,IV:4,I:1}, timi = "",i;
for (i in lookupTable) {
while (num >= lookupTable[i]) {
timi += i;
num -= lookupTable[i];
}
}
return timi;
}
console.log(convertToRoman(3999));