Typescript, How to write a typed flatten method for array

Question

I am trying to write in typescript something similar to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat

However, I want to be able to flatten nested list of arbitrary depth; and also I want to restrict all non array elements to a single type.

Something like:

interface NestedList<T> extends Array<T | NestedList<T>> {}

I have got this so far:

export const flat = <T>(ls: NestedList<T>): T[] => {
   const reducer = (acc: T[], it: T | NestedList<T>): T[] => acc.concat(
      Array.isArray(it) ? it.reduce(reducer, []) : it
   );

   return ls.reduce(reducer, []);
};

However, the type inference does not seem to be working

   it('works with type inference', () => {
      interface Foo<T> { v: T; }

      const data: { [_: string]: { [_: string]: Foo<number> } } = {
         bar: { x: { v: 1 } },
         moo: { y: { v: 2 }, z: { v: 3 } }
      };

      const nested: Foo<number>[][] = Object.values(data).map(val => Object.values(val));
      const list2: Foo<number>[] = flat(nested);
      expect(list2).toEqual([{ v: 1 }, { v: 2 }, { v: 3 }]);
   });

It receives type error on this line:

const list2: Foo<number>[] = flat(nested);

Error:

Type '(Foo<number>[] | ConcatArray<Foo<number>[]>)[]' is not assignable to type 'Foo<number>[]'.
  Type 'Foo<number>[] | ConcatArray<Foo<number>[]>' is not assignable to type 'Foo<number>'.
    Property 'v' is missing in type 'Foo<number>[]' but required in type 'Foo<number>'

What I am missing here? Any help is appreciated.

Answer 1

I stumbled on the same problem as you. This is the solution I came up with:

function flat<U>(arr: U[][]): U[] {
    if ((arr as unknown as U[]).every((val) => !Array.isArray(val))) {
        return (arr as unknown as U[]).slice();
    }
    return arr
        .reduce((acc, val) => acc
            .concat(Array.isArray(val) ? flat((val as unknown as U[][])) : val), []);
}

console.log(flat([[1,2],[[[3]]],4])

Hope it helps you :)