### partition

Groups the elements into two arrays, depending on the provided function's truthiness for each element. 

Use `Array.prototype.reduce()` to create an array of two arrays. 
Use `Array.prototype.push()` to add elements for which `fn` returns `true` to the first array and elements for which `fn` returns `false` to the second one. 

```js
const partition = (arr, fn) =>
  arr.reduce(
    (acc, val, i, arr) => {
      acc[fn(val, i, arr) ?  0 : 1].push(val);
      return acc;
    },
    [[], []]
  );
```

```js
const users = [{ user: 'barney', age: 36, active: false }, { user: 'fred', age: 40, active: true }];
partition(users, o => o.active); // [[{ 'user': 'fred',    'age': 40, 'active': true }],[{ 'user': 'barney',  'age': 36, 'active': false }]]
```
