Question: is there a more elegant way to representing type Line = string | Line[]?

[zpdDG4gta8XKpMCd]

I wish I could define a type as follows

type Line = string | Line[]; // <-- not a real TypeScript, do not attempt at home!

Looks like it's there is a fundamental limitation in the type system that doesn't allow doing so.

I ended up having my type defined like this:

export type Line = string|(string|(string|(string|(string|(string|(string|(string|(string|(string|(string|(string|string|(string|(string|(string|(string|(string|(string|(string|(string|(string|(string|(string|string|(string|(string|(string|(string|(string|(string|(string|(string|(string|(string|(string|string[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[])[];

but I don't like it because it only works that many times

Is there a more elegant and typesafe way of getting a type like this that works beyond 27 nested levels?

[RyanCavanaugh]

I don't think we have a way to represent an infinitely-expanding type alias like this.

[RyanCavanaugh]

A shorter and sillier way of writing this type to depth 10

let x = ['', ['', ['', ['', ['', ['', ['', ['', ['', ['']]]]]]]]]];
type Line = string|typeof x;

[zpdDG4gta8XKpMCd]

+1 i like it

[danquirk]

I think #3496 covers your initial request.

[jeffreymorlan]

type Line = string | LineArray;
interface LineArray extends Array<Line> {}

[RyanCavanaugh]

@jeffreymorlan 🏆 👍!

[zpdDG4gta8XKpMCd]

whose name shall it put on the check?

[mhegazy]

Looks like the original issue has been answered. please reactivate if you are running into other issues.

[davidcornu]

There's an interesting limitation to the suggestion in #3988 (comment).

Given

type ItemOrArray<T> = T | ItemArray<T>;
interface ItemArray<T> extends Array<ItemOrArray<T>> { }

the following

const items: Array<ItemOrArray<number>> = [1, [2, [3, ["4"]]]];

will report an error

Type '(number | (number | (number | string[])[])[])[]' is not assignable to type 'ItemOrArray<number>[]'.
  Type 'number | (number | (number | string[])[])[]' is not assignable to type 'ItemOrArray<number>'.
    Type '(number | (number | string[])[])[]' is not assignable to type 'ItemOrArray<number>'.
      Type '(number | (number | string[])[])[]' is not assignable to type 'ItemArray<number>'.
        Types of property 'push' are incompatible.
          Type '(...items: (number | (number | string[])[])[]) => number' is not assignable to type '(...items: ItemOrArray<number>[]) => number'.
            Types of parameters 'items' and 'items' are incompatible.
              Type 'ItemOrArray<number>' is not assignable to type 'number | (number | string[])[]'.
                Type 'ItemArray<number>' is not assignable to type 'number | (number | string[])[]'.
                  Type 'ItemArray<number>' is not assignable to type '(number | string[])[]'.
                    Types of property 'push' are incompatible.
                      Type '(...items: ItemOrArray<number>[]) => number' is not assignable to type '(...items: (number | string[])[]) => number'.
                        Types of parameters 'items' and 'items' are incompatible.
                          Type 'number | string[]' is not assignable to type 'ItemOrArray<number>'.
                            Type 'string[]' is not assignable to type 'ItemOrArray<number>'.
                              Type 'string[]' is not assignable to type 'ItemArray<number>'.
                                Types of property 'push' are incompatible.
                                  Type '(...items: string[]) => number' is not assignable to type '(...items: ItemOrArray<number>[]) => number'.
                                    Types of parameters 'items' and 'items' are incompatible.
                                      Type 'ItemOrArray<number>' is not assignable to type 'string'.
                                        Type 'number' is not assignable to type 'string'.

but

const items: Array<ItemOrArray<number>> = [1, [2, [3, [4, ["5"]]]]];

will not.

Edit - this seems related to #13052.