countBy - 그룹별 카운트
const countBy = (arr, fn) => arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => { acc[val] = (acc[val] || 0) + 1; return acc; }, {}); countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2} countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}
