groupBy - 그룹별 함수 실행
const groupBy = (arr, fn) => arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => { acc[val] = (acc[val] || []).concat(arr[i]); return acc; }, {}); groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]} groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']}