turn flat structure into nested structure

getNestedItems.ts

type Item = {
	id: string
	parentId: string | undefined
}

type NestedItem = Item & {
	children: NestedItem[]
}

const getNestedItems = (items: Item[], parentId: string | undefined = undefined) => {
	return items
		.filter((x) => x.parentId === parentId)
		.map<NestedItem>((x) => ({ ...x, children: getNestedItems(items, x.id) }))
}

console.log(JSON.stringify(getNestedItems([
	{ id: '1', parentId: undefined },
	{ id: '2', parentId: '1' },
	{ id: '3', parentId: '2' },
	{ id: '4', parentId: '1' },
]), null, 4))