array-to-tree.js

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

let input = [
  {
    id: 1,
    City: "Coevorden",
    Name: "Grady"
  },
  {
    id: 2,
    parentId: 1,
    City: "Chełm",
    Name: "Scarlet"
  }
];

createDataTree(input);