Simple JavaScript Group

group.js

"use strict";

let group = (arr, by) => arr.reduce((prev, cur) => {
        let val = isFunction(by) ? by(cur) : cur[by];
        prev.hasOwnProperty(val) ? prev[val].push(cur) : prev[val] = [cur];
        return prev;
    }, {}),
    isFunction = (x) => Object.prototype.toString.call(x) === "[object Function]";

module.exports = () => ({group, isFunction});