> If it's correct, it's not a maintenance nightmare, and it will alert you to problems later when someone wants to use it incorrectly.

You're confusing things. It is a maintenance nightmare because it is your job to ensure it is correct and remains correct in spite of changes. You are the one owning that mess and held accountable for it.

> If you're writing first-party software, it probably doesn't matter. But if you have consumers, it's important.

Yes, it is important that you write correct and usable code. That code doesn't fall on your lap though and you need to be the one writing and maintaining it. Whoever feels compelled to write unintelligible character soup that makes even experienced seasoned devs pause and focus is failing their job as a software engineer.

The alternative is what shows in the comment: go on HN and tell the world you think TS and JS are crap and it's not worth your time, while writing poor software.

To answer this we probably need more details, otherwise it's gonna be an XY Problem. What is it that I'm trying to do? How would I type this function in, say, SML, which isn't going to allow incorrect types but also doesn't allow these kinds of type gymnastics?

For one, the simple answer is incomplete. It gives the fully unwrapped type of the array but you still need something like

  type FlatArray<T extends unknown[]> = Flatten<T[number]>[]

First one flattens a potentially-nested tuple type. E.g., FlatArr<[number, [boolean, string]]> is [number, boolean, string].

Second one gets the element type of a potentially-nested array type. E.g., Flatten<number[][]> is number.

For what it's worth, I've never needed to use either of these, though I've occasionally had other uses for slightly fancy TypeScript type magic.

The answer above actually gets the type union of all non-array elements of a multi-level array.

In other words

    Flatten<[1,[2,'a',['b']]]>

    const foo: Flatten<[1,[2,'a',['b']]]> = 'b'; // OK
    const bar: Flatten<[1,[2,'a',['b']]]> = 'c'; // Error: Type '"c"' is not assignable to type '1 | 2 | "a" | "b"'

     type Flatten<T> = T extends Array<unknown> ? Flatten<T[number]> : T

My solution was for flattening an array with a depth of 1 (most people using Array.prototype.flat are using this default depth I'd wager):

    console.log(JSON.stringify([1,[2, [3]]].flat()));
    > [1,2,[3]]

    // 'readonly' added to the 'extends' sections to work on "as const" (readonly) arrays
    type FlatArr<Arg extends unknown[] | readonly unknown[]> = Arg extends readonly [infer First, ...(infer Rest)] ?
      First extends readonly unknown[] ?
        [...First, ...FlatArr<Rest>] :
        [First, ...FlatArr<Rest>] :
      [];

    const flatten = <Arr extends unknown[] | readonly unknown[]>(arr: Arr): FlatArr<Arr> => arr.flat() as FlatArr<Arr>
    const someArr = [1,[2,'a',['b', ['c']]]] as const; // const someArr: readonly [1, readonly [2, "a", readonly ["b", readonly ["c"]]]]
    const someArr2 = flatten(someArr);                 // const someArr2: [1, 2, "a", readonly ["b", readonly ["c"]]]

    

I'd call that bad pretty bad.

Without internet or AI I wouldn't attempt writing anything like that.

Java: List<Object>

Python: list[Any]

...what am I missing?