jsは配列をツリー構造に変換します.

[{
  //       
  name: Id,
  children: [Id],
}]

const arryData = [
  { name: 1, children: [ 2, 3 ] },
  { name: 2, children: [ 4, 5 ] },
  { name: 4, children: [ 7 ] },
  { name: 7, children: [ 8, 9 ] },
  { name: 8, children: [] },
  { name: 9, children: [] },
  { name: 5, children: [] },
  { name: 3, children: [ 6 ] },
  { name: 6, children: [] }
]

const treeStructure = {
  name: Id, // name Id，   Number, String.    
  children: [treeStructure], // treeStructure   
  //       
}
  

{
  name: 1,
  children: [{
    name: 2,
    children: [{
      name: 4,
      children: [{
        name: 7,
        children: [{
          name: 8,
          children: []
        }, {
          name: 9,
          children: []
        }]
      }],
    }, {
      name: 5,
      children: [],
    }]
  }, {
    name: 3,
    children: [{
      name: 6,
      children: [],
    }],
  }],
}

const arryData = [
  { name: 1, children: [ 2, 3 ] },
  { name: 2, children: [ 4, 5 ] },
  { name: 4, children: [ 7 ] },
  { name: 7, children: [ 8, 9 ] },
  { name: 8, children: [] },
  { name: 9, children: [] },
  { name: 5, children: [] },
  { name: 3, children: [ 6 ] },
  { name: 6, children: [] }
]

//     transferArrayToTree  ,  arrayData   ,      treeData    

// 2.      arrayData[idx]    
//     return {
//       name: arrayData[idx].name,
//       children: [transferArrayToTree(idx, arrayData)]
//     }

//     ，         
function transferArrayToTree(idx, arrayData) {
  let children = [], root = {}
  if (idx >= 0) {
    // console.log(idx)
    root.name = arrayData[idx].name
    root.children = arrayData[idx].children.map(value => {
      return transferArrayToTree(
        findChildrenIdxOfArray(value, arrayData),
        arrayData
      )
    })
    return root
  } else {
    return {}
  }
}

// name   
function findChildrenIdxOfArray(name, arrayData) {
  return arrayData.findIndex(value => {
    return value.name === name
  })
}

//     name  children   
function findRootIdxOfArray(arrayData) {
  let childrenNames = arrayData.reduce((pre, now) => {
    pre = pre.concat(now.children)
    return pre
  }, [])
  return arrayData.findIndex(value => {
    return childrenNames.indexOf(value.name) < 0
  })
}

console.log('root idx', findRootIdxOfArray(arrayData))
const treeDataCopy = transferArrayToTree(0, arrayData)
console.log(treeDataCopy)