```
var longestPalindrome = function(s) {
const counts = {};
for (let c of s) {
counts[c] = (counts[c] || 0) + 1;
}
let odds = 0;
for (let c in counts) {
odds += counts[c] % 2;
}
return s.length - odds + !!odds;
};
```

And here's a contrived one-liner:

```
var longestPalindrome = function(s, o) {
return s.length - (o = Object.values(s.split('').reduce((c, a) => (c[a] = (c[a] || 0) + 1, c), {})).filter(a => a % 2).length) + !!o;
};
```