nest

Nests recursively objects linked to one another in a flat array.

• Use recursion.
• Use Array.prototype.filter() to filter the items where the id matches the link.
• Use Array.prototype.map() to map each item 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.

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

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: [...] }]