In this challenge, you would need to write a type that takes an array and emitted the flatten array type.
For example:
type flatten = Flatten<[1, 2, [3, 4], [[[5]]]]> // [1, 2, 3, 4, 5]
import type { Equal, Expect } from '@type-challenges/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']>>,
]
// @ts-expect-error
type error = Flatten<'1'>