### nest

Given a flat array of objects linked to one another, it will nest them recursively.
Useful for nesting comments, such as the ones on reddit.com.

Use recursion. 
Use `Array.filter()` to filter the items where the `id` matches the `link`, then `Array.map()` to map each one to a new object that has a `children` property which recursively nests the items based on which ones are children of the current item. 
Omit the second argument, `id`, to default to `null` which indicates the object is not linked to another one (i.e. it is a top level object). 
Omit the third argument, `link`, to use `'parent_id'` as the default property which links the object to another one by its `id`.

```js
const nest = (items, id = null, link = 'parent_id') =>
  items
    .filter(item => item[link] === id)
    .map(item => ({ ...item, children: nest(items, item.id) }));
```

```js
// One top level comment
const comments = [
  { id: 1, parent_id: null },
  { id: 2, parent_id: 1 },
  { id: 3, parent_id: 1 },
  { id: 4, parent_id: 2 },
  { id: 5, parent_id: 4 }
];
const nestedComments = nest(comments); // [{ id: 1, parent_id: null, children: [...] }]
```