noone182

const result = [...subcategory.reduce(
  (acc, n) => (acc.get(n.category)?.children.push(n), acc),
  new Map(category.map(n => [ n.id, { ...n, children: [] } ]))
).values()];

const result = category.map(n => ({
  ...n,
  children: subcategory.filter(m => m.category === n.id),
}));

const result = category.map(function(n) {
  return {
    ...n,
    children: this[n.id] ?? [],
  };
}, subcategory.reduce((acc, n) => ((acc[n.category] ??= []).push(n), acc), {}));