00459-medium-flatten.ts

// ============= Test Cases =============
import type { Equal, Expect } from "./test-utils";

type cases = [
    Expect<Equal<Flatten<[]>, []>>,
    Expect<Equal<Flatten<[1, 2, 3, 4]>, [1, 2, 3, 4]>>,
    Expect<Equal<Flatten<[1, [2]]>, [1, 2]>>,
    Expect<Equal<Flatten<[1, 2, [3, 4], [[[5]]]]>, [1, 2, 3, 4, 5]>>,
    Expect<
        Equal<
            Flatten<[{ foo: "bar"; 2: 10 }, "foobar"]>,
            [{ foo: "bar"; 2: 10 }, "foobar"]
        >
    >
];

// ============= Your Code Here =============
type Flatten<T extends unknown[]> = T extends [infer First, ...infer Rest]
    ? First extends unknown[]
        ? [...Flatten<First>, ...Flatten<Rest>]
        : [First, ...Flatten<Rest>]
    : T;