I want describe some data as where a value in the array could be another array of numbers, this is done indefinitely so the types should be recursive.
With the following code I receive an error
Type 'number' is not assignable to type 'Data'
type Data = ReadonlyArray<number> | Data[];
const data: Data = [1, 2, 3, [4, 5, [6, 7]]];
How to fix it using the latest TypeScript?
Typescript treats an array type of unions and unions of array very differently. Something like [1, 2, 3, [4, 5]] is actually of type (number | number[])[] - which is not assignable to number[] | number[][].
This means that you need an array of unions, where the union can circularly reference itself.
type Data = readonly (number | Data)[];
The above will declare a readonly array of the recursive atom, number. Now you'll be able to assign [1, 2, 3, [4, 5, [6, 7]]]
Try it out on playground
type Data = ReadonlyArray<number | Data>; ? WDYT?
readonly number[] is the same as ReadonlyArray<number> - readonly as a keyword is just more general, you can use ReadonlyArray<number | Data> just fine.type E = number
type ListOfE = E[]
type NestedListOfE = (E | NestedListOfE)[]
const a: E = 10
const b: ListOfE = [ 1, 2, 3 ]
const c: NestedListOfE = [ 1, 2, [ 3, [ 4, 5 ] ] ]
Note usage of recursive types is only available since TS 3.7.
