Performatic list to tree using one loop and map

list-to-tree.js

//+ Jonas Raoni Soares Silva
//@ http://raoni.org

function tree(items) {
  const tree = [];
  const map = new Map;
  for (const item of items) {
    const placeholder = map.get(item.id);
    item.children = placeholder ? placeholder.children : [];
    map.set(item.id, item);

    //root condition
    if (item.parentId === item.id) {
      tree.push(item);
    }
    else {
      const parent = map.get(item.parentId);
      if (!parent) {
        map.set(item.parentId, {children: [item]})
      }
      else {
        parent.children.push(item);
      }
    }
  }
  return tree;
}