### averageBy

Returns the average of an array, after mapping each element to a value using the provided function.

Use `Array.map()` to map each element to the value returned by `fn`, `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`, divide by the `length` of the array.

```js
const averageBy = (arr, fn) => arr.map(fn).reduce((acc, val) => acc + val, 0)/arr.length;
```

```js
averageBy([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], o => o.n); // 5
```