Maketree for recursion episode, more functional and with better time complexity

maketree.js

const createChildLookupTable = categories => 
  categories.reduce((lookup, child) => 
    Object.assign({}, lookup, {
      [child.parent]: (lookup[child.parent] || []).concat(child)
    }), {})

const makeTree = (childLookup, parent) => 
  (childLookup[parent] || []).reduce((node, child) => 
    Object.assign({}, node, {
  	  [child.id]: makeTree(childLookup, child.id)
    }), {})

console.log(makeTree(createChildLookupTable([
  { id: 'animals', parent: 'root' },
  { id: 'dogs', parent: 'animals' },
  { id: 'cats', parent: 'animals' },
  { id: 'chiuaua', parent: 'dogs' },
  { id: 'labrador', parent: 'dogs' },
  { id: 'siamese', parent: 'cats' },
  { id: 'munchkin', parent: 'cats' }
]), 'root')) 

v nice.