Дима Паутов

function createTree(arr, structure) {
  const tree = Object.fromEntries(arr.map(n => [ n.id, { ...n } ]));
  const nonRootIds = structure.flatMap(n => n.children);

  structure.forEach(n => tree[n.id].elements = n.children.map(m => tree[m]));

  return Object.values(tree).filter(n => !nonRootIds.includes(n.id));
}

const result = array
  .concat(obj)
  .reduceRight((acc, n) => ({
    ...n,
    children: acc ? [ acc ] : [],
    // или, если вдруг исходные массивы непустые,
    // и их содержимое надо сохранить
    // children: n.children.concat(acc || []),
  }), null);

const result = [ ...array, obj ].reduceRight(
  (acc, { children, ...n }) => (n.next = acc, n),
  null
);

const getById = id => data.filter(s => s.structureId === id)[0] || false;
const getChild = me => data.filter(s => s.parentId === me.structureId)[0] || false;
const getMyParent = me => getById(me.parentId);

const getParentsChain = me => {
  const chain = [];
  let parent = getMyParent(me);
  while(parent) {
    chain.unshift(parent);
    parent = getMyParent(parent);
  }
  return chain;
}

const getChildrenChain = me => {
  const chain = [];
  let child = getChild(me);
  while(child) {
  	chain.push(child);
    child = getChild(child);
  }
  return chain;
}

// test
const me = getById(3);
console.log([...getParentsChain(me), me, ...getChildrenChain(me)]); /*
0: Object { name: "Дирекция юкрк", parentId: null, structureId: 1 }
​1: Object { name: "Южно-Курильск", parentId: 1, structureId: 2 }
​2: Object { name: "Служба главного инженера", parentId: 2, structureId: 3 }
​3: Object { name: "Мехцех", parentId: 3, structureId: 4 } */