Try It Yourself - SyntaxStudy Editor

HTML / CSS / JS

// Recursive JSON type
type JSONPrimitive = string | number | boolean | null;
type JSONObject    = { [key: string]: JSONValue };
type JSONArray     = JSONValue[];
type JSONValue     = JSONPrimitive | JSONObject | JSONArray;

const data: JSONValue = {
    name: "Alice",
    scores: [98, 87, 76],
    address: { city: "London", zip: "EC1A 1BB" },
    active: true,
};

// Recursive tree structure
type TreeNode<T> = {
    value: T;
    children: TreeNode<T>[];
};

function sumTree(node: TreeNode<number>): number {
    return node.value + node.children.reduce(
        (acc, child) => acc + sumTree(child),
        0
    );
}

const tree: TreeNode<number> = {
    value: 1,
    children: [
        { value: 2, children: [{ value: 4, children: [] }] },
        { value: 3, children: [] },
    ],
};

console.log(sumTree(tree)); // 10

// Recursive readonly deep type
type DeepReadonly<T> = T extends (infer U)[]
    ? readonly DeepReadonly<U>[]
    : T extends object
    ? { readonly [K in keyof T]: DeepReadonly<T[K]> }
    : T;

type Config = { server: { host: string; port: number }; debug: boolean };
type FrozenConfig = DeepReadonly<Config>;