W3Schools Tryit Editor

// Linked list
type Linked<T> = { value: T; next: Linked<T> | null };
const list: Linked<number> = { value: 1, next: { value: 2, next: { value: 3, next: null } } };
function listValues<T>(l: Linked<T> | null): T[] {
  const out: T[] = [];
  while (l) {
    out.push(l.value);
    l = l.next;
  }
  return out;
}
console.log(listValues(list).join(" -> "));

// Directory tree
type File = { type: "file"; name: string; size: number };
type Directory = { type: "directory"; name: string; children: (File | Directory)[] };
const fs: Directory = {
  type: "directory",
  name: "root",
  children: [{ type: "file", name: "a.txt", size: 10 }, { type: "directory", name: "sub", children: [] }],
};
function countFiles(node: File | Directory): number {
  if (node.type === "file") return 1;
  return node.children.reduce((n, child) => n + countFiles(child), 0);
}
console.log(countFiles(fs));

// Recursive function type (runtime)
type RecursiveFunction<T> = (x: T | RecursiveFunction<T>) => void;
const f: RecursiveFunction<number> = (x) => {
  if (typeof x === "number") console.log(x);
};
f(42);