unflatten-array.ts

/**
 * Unflatten an array of objects.
 * 
 * @param arr An array of objects.
 * @param keyFn Key property accessor function.
 * @param parentFn Parent property accessor function.
 * @example unflatten<FlatTreeNode>(x, i => i.id, i => i.parentId);
 */
private unflatten<T>(arr: T[], keyFn: (item: T) => number, parentFn: (item: T) => number): T[] {
  const tree = [],
    map: Map<number, T> = new Map(arr.map(i => [keyFn(i), i]));

  map.forEach(it => {
    if (parentFn(it) === 0) {
      tree.push(it);
    } else {
      let parent = map.get(parentFn(it));

      if (parent['children']) {
        parent['children'].push(it);
      } else {
        parent['children'] = [it];
      }
    }
  });

  return tree;
}