make tree from flatten array

tree_from_flatten_arr.js

const createDataTree = dataset => {
    let hashTable = Object.create(null)
    dataset.forEach( aData => hashTable[aData.id] = { ...aData, childNodes : [] } )
    let dataTree = []
    dataset.forEach( aData => {
      if( Number.isInteger(aData.parentId)) {
          hashTable[aData.parentId].childNodes.push(hashTable[aData.id])
        } else {
          dataTree.push(hashTable[aData.id])
        }
    } )
    return dataTree
}