const countBy = (arr, fn) =>
arr.map(typeof fn === "function" ? fn : val => val[fn]).reduce((acc, val, i) => {
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} my question is how it works in map. arr.map () is mainly because the result of this ternary expression should be entered into fn , rather than the following reduce asking the boss to focus on explaining this