```
var romanToInt = function(s) {
let dictionary = {
"I": 1,
"V": 5,
"X": 10,
"L": 50,
"C": 100,
"D": 500,
"M": 1000,
};
if (typeof s !== "string") {
return null;
}
if (s.length === 1) {
return dictionary[s];
}
let res = 0;
for (let i = 0; i < s.length - 1; i++) {
if (dictionary[s[i + 1]] > dictionary[s[i]]) {
res -= dictionary[s[i]];
} else {
res += dictionary[s[i]];
}
}
return res + dictionary[s[s.length - 1]];
};
Suggestions always appreciated!
```