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

Use recursion.
Use Array.prototype.filter() to filter the items where the id matches the link, then 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.


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


// 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: [...] }]