### findLastIndex

Returns the index of the last element for which the provided function returns a truthy value.

Use `Array.map()` to map each element to an array with its index and value.
Use `Array.filter()` to remove elements for which `fn` returns falsey values, `Array.slice(-1)` to get the last one.

```js
const findLastIndex = (arr, fn) =>
  arr
    .map((val, i) => [i, val])
    .filter(val => fn(val[1], val[0], arr))
    .slice(-1)[0][0];
```

```js
findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)
```