Level 4: Data shaping · Exercise 59 of 76 · Free

Flat list to a tree

A JavaScript exercise from Scriptdrill. Solve it with a code editor in your browser that runs your function against example and hidden tests, and it is checked the moment you submit.

The task

Each item looks like `{ id, parentId, name }`, where `parentId` is the `id` of another item, or `null` for a top-level item. Return the top-level items as an array of nodes `{ id, name, children }`, where `children` holds the node's child nodes in the same shape (an empty array when there are none). Nodes keep the order of the input list, at every level. A child can appear in the list before its parent. Every `parentId` that is not `null` refers to an item in the list. Do not change the items.

What this exercise teaches

Creating all the nodes before linking any of them is what makes the order of the list irrelevant: by the second pass every parent already exists in the Map. Doing both in one pass only works when parents happen to come first.

A hint if you get stuck

Make two passes. First create a node for every item and store it in a `Map` by id. Then go through the items again and push each node into its parent's `children`, or into the result when `parentId` is `null`.