Single-pass tree structure

single-pass-tree.js

function createTree(nodes) {
	const children = {};
	const tree     = [];

	for(const node of nodes) {
		node.children = children[node.id] || (children[node.id] = []);
		(node.parent_id ? children[node.parent_id] || (children[node.parent_id] = []) : tree).push(node);
	}

	return tree;
}

var nodes = [
	{
		id        : 1,
		parent_id : null,
	},
	{
		id        : 2,
		parent_id : 1,
	},
	{
		id        : 3,
		parent_id : 4,
	},
	{
		id        : 4,
		parent_id : 2,
	},
	{
		id        : 5,
		parent_id : 2,
	}
];

console.log(JSON.stringify(createTree(nodes), null, 2));