### pickBy

Creates an object composed of the properties the given function returns truthy for. The function is invoked with two arguments: (value, key).

Use `Object.keys(obj)` and `Array.filter()`to remove the keys for which `fn` returns a falsey value.
Use `Array.reduce()` to convert the filtered keys back to an object with the corresponding key-value pairs.

```js
const pickBy = (obj, fn) =>
  Object.keys(obj)
    .filter(k => fn(obj[k], k))
    .reduce((acc, key) => ((acc[key] = obj[key]), acc), {});
```

```js
pickBy({ a: 1, b: '2', c: 3 }, x => typeof x === 'number'); // { 'a': 1, 'c': 3 }
```