Recursive Types

A recursive type can describe trees and nested documents by referring to itself. Recursive data structures need a terminating case. Avoid unbounded recursion on untrusted input, because JavaScript still has a finite call stack.

Reading the code: The type permits TreeNode[] inside a Node. At runtime count starts with one for the parent and recursively adds one for each leaf, giving three nodes.

Keep the boundary clear: Recursive types do not prevent cyclic object graphs. If cycles are possible, add a visited-set or depth limit to traversal.

Save the code as recursive-types.ts. The result below was captured after compiling this exact example with strict TypeScript checking and executing the emitted JavaScript in Node.js.

TypeScript code

type TreeNode = { label: string; children?: TreeNode[] };
function count(node: TreeNode): number {
  return 1 + (node.children ?? []).reduce((sum, child) => sum + count(child), 0);
}
const tree: TreeNode = { label: "root", children: [{ label: "a" }, { label: "b" }] };
console.log(count(tree));

Verified output

3

Build: npx tsc --strict --target es2022 --module commonjs --lib es2022,dom,esnext.disposable recursive-types.ts; run: node recursive-types.js. The exact filename may be changed freely. When a multi-file project exists, prefer its tsconfig.json and build scripts.

The type permits TreeNode[] inside a Node. At runtime count starts with one for the parent and recursively adds one for each leaf, giving three nodes.

The output shown above comes from the initialized values in this program. Replacing a type annotation alone does not transform a JavaScript value.

Recursive types do not prevent cyclic object graphs. If cycles are possible, add a visited-set or depth limit to traversal.

Check both the TypeScript diagnostic and the emitted JavaScript behavior. A program can compile correctly and still need runtime validation or error handling.

Add a grandchild under a and predict count(tree) before running it.

Before running your change, write down the type you expect TypeScript to infer and the output you expect JavaScript to print. Then check both predictions.

A recursive type can describe trees and nested documents by referring to itself.

Try the practice change and re-run the exact program to test your understanding.