Lesson 23 — Recursive types

Types that contain themselves

A recursive type refers to itself, which is how you describe trees: a post has replies, and each reply is a post with its own replies. Code for such data is usually recursive too — a function that calls itself for each child until it reaches items with no children.

type Folder = { name: string; children: Folder[] };
function depth(f: Folder): number {
  return 1 + Math.max(0, ...f.children.map(depth));
}

Why it matters: comments, menus, file systems and JSON are all trees.