const maxKey = function (o) {
const [r] = Object.entries(o).reduce(([r, max], [k, v]) => {
switch (Math.sign(v - max)) {
case -1:
return [r, max];
case 0:
return r.push(k), [r, max];
case 1:
return [[k], v];
}
});
return r;
};