A function that flattens a tree structure while preserving nesting information

levelizeTree.js

const { map, flatMapDeep, add, flow } = require('lodash/fp')

const data = [
  {
    id: 1,
    type: 'group',
    name: 'Colors',
    items: [
      { id: 2, type: 'item', name: 'Red' },
      { id: 3, type: 'item', name: 'Purple' },
      { id: 4, type: 'item', name: 'Black' }
    ]
  },
  {
    id: 5,
    type: 'group',
    name: 'Food',
    items: [
      { id: 6, type: 'item', name: 'Cheese' },
      { id: 7, type: 'item', name: 'Potatoes' },
      { id: 10,
        type: 'group',
        name: 'Sandwich',
        items: [
          { id: 11, type: 'item', name: 'Bread' },
          { id: 12, type: 'item', name: 'Cheese' },
          { id: 13, type: 'item', name: 'Salami' }
        ] }
    ]
  },
  { id: 8, type: 'item', name: 'Kitten' },
  {
    id: 14,
    type: 'group',
    name: 'Matryoshka A',
    items: [
      {
        id: 15,
        type: 'group',
        name: 'Matryoshka B',
        items: [
          {
            id: 16,
            type: 'group',
            name: 'Matryoshka C',
            items: [
              {
                id: 17,
                type: 'group',
                name: 'Matryoshka D',
                items: [
                  {
                    id: 18,
                    type: 'item',
                    name: 'Secret Treasure'
                  }
                ]
              }
            ]
          }
        ]
      }
    ]
  },
  { id: 9, type: 'item', name: 'Rainbow' },
  {
    id: 19,
    type: 'group',
    name: 'Box',
    items: [
      {
        id: 20,
        type: 'group',
        name: 'Box in a box (which is empty)',
        items: []
      }
    ]
  }
]

const increment = add(1)

const levelizeIterator = childrenProperty => levelProperty => function iterateLevel (level = 0) {
  return item => {
    item[levelProperty] = level

    if (item.hasOwnProperty(childrenProperty)) {
      const step = flow(increment, iterateLevel, flatMapDeep)
      const nextLevel = step(level)
      const children = nextLevel(item[childrenProperty])
      delete item[childrenProperty]
      return [item, children]
    }

    return item
  }
}

// The children of a node are stored in a property named `items`
// The level information will be stored in a property named `level`
const itemsLevelizer = levelizeIterator('items')('level')

// The `level` property is mainly used interlally. You can set it to whatever you need.
// It defaults to 0
const rootItemsLevelizer = itemsLevelizer()

const levelizeTree = flatMapDeep(rootItemsLevelizer)
const dataFlat = levelizeTree(data)

const logJSON = flow(JSON.stringify, console.log)
logJSON(dataFlat)

const stringifyItem = item => `${'  '.repeat(item.level)}[${item.id}] ${item.name} ${item.type === 'group' ? '(+)' : ''}`
const logItem = flow(stringifyItem, console.log)
const logItems = map(logItem)
logItems(dataFlat)