Algorithm to transform array into tree

arrayTotree.ts

export type Node = { id: number; parentId: number }

const nodes: Node[] = [
  { id: 1, parentId: null },
  { id: 2, parentId: 1 },
  { id: 3, parentId: 1 },
  { id: 4, parentId: 2 },
  { id: 5, parentId: 2 },
  { id: 6, parentId: 3 },
  { id: 7, parentId: 3 }
]

const arrayToTree = (nodes: Node[]) => {
  const lookup = {}
  const rootNodes = []
  for (const node of nodes) {
    if (!lookup[node.id]) lookup[node.id] = { ...node, children: [] } // init
    lookup[node.id] = { ...lookup[node.id], ...node } // update

    const treeNode = lookup[node.id] // reference

    const parentId = treeNode.parentId

    if (!parentId) rootNodes.push(treeNode)

    if (!lookup[parentId]) lookup[parentId] = { children: [] }

    lookup[parentId].children.push(treeNode)
  }

  return rootNodes
}

console.log(JSON.stringify(arrayToTree(nodes), null, 2))