groupBy for javascript

groupBy.js

function groupBy(xs, key) {
    return xs.reduce(function (rv, x) {
        let v = key instanceof Function ? key(x) : x[key];
        let el = rv.find((r) => r && r.key === v);
        if (el) {
            el.values.push(x);
        } else {
            rv.push({
                key: v,
                values: [x]
            });
        }
        return rv;
    }, []);
}