30-seconds-of-code/snippets/nest.md

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 use 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.

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

// 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: [...] }]