Generate a tree from flatten array of data in Javascript

tree.js

function generateTree(list, id = 'slug', parentId = 'parent_slug', childrenId = 'children') {
  var tree = [];
  var lookup = {};

  list.forEach(function(item) {
    lookup[item[id]] = item;
    item[childrenId] = [];
  });
  list.forEach(function(item) {
    if (item[parentId] != null) {
	  lookup[item[parentId]][childrenId].push(item);
	}
	else {
	  tree.push(item);
	}
  });
  return tree;
}