treeify

treeify.js

const assert = require('assert')

// takes a flat structure like this
const input = [
  {
    level: 1,
    title: '1'
  },
  {
    level: 1,
    title: '2'
  },
  {
    level: 2,
    title: '2 1'
  },
  {
    level: 3,
    title: '2 1 1'
  },
  {
    level: 2,
    title: '2 2'
  },
  {
    level: 1,
    title: '3'
  },
  {
    level: 2,
    title: '3 1'
  },
  {
    level: 1,
    title: '4'
  }
]
// And converts it into this
const expected = [
  {
    level: 1,
    title: '1',
    children: []
  },
  {
    level: 1,
    title: '2',
    children: [
      {
        level: 2,
        title: '2 1',
        children: [
          {
            level: 3,
            title: '2 1 1',
            children: []
          }
        ]
      },
      {
        level: 2,
        title: '2 2',
        children: []
      }
    ]
  },
  {
    level: 1,
    title: '3',
    children: [
      {
        level: 2,
        title: '3 1',
        children: []
      }
    ]
  },
  {
    level: 1,
    title: '4',
    children: []
  }
]

function treeify(list) {
  if (list.length === 0) {
    return []
  }
  const [head, ...tail] = list
  const nextIndex = tail.findIndex(item => item.level <= head.level)

  return [
    setChildren(head, treeify(tail.slice(0, nextIndex))),
    ...treeify(tail.slice(nextIndex))
  ]
}

function setChildren(item, children) {
  return {
    ...item,
    children
  }
}

const actual = treeify(input)

assert.deepEqual(actual, expected)