Is there a way to recursively unwrap a function type in TypeScript?

user_2417344 0
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Let's say we have the following type I:

type I = () => () => () => "a" | "b" | "c";

Is there a way to create a generic type Unwrap such that Unwrap<I> evaluates to "a" | "b" | "c"?

type I = () => () => () => "a" | "b" | "c";

type Result = Unwrap<I>; // "a" | "b" | "c"

The following (ofc) produces a circularity error:

type Unwrap<
  T extends (...args: any[]) => any,
  R = ReturnType<T>
> = R extends (...args: any[]) => any
  ? Unwrap<R>
  : R;

Any help would be greatly appreciated. Thank you!

inferred-type unwrap typescript compiler-errors types
2 Answers

Well here's the hack which works in TypeScript 3. It's actually not that awful.

type I = () => (() => (() => "a" | "b" | "c")) | "e" | (() => "f" | "g");

type Unwrap<T> =
    T extends (...args: any[]) => infer R
        ? { 0: Unwrap<R> }[T extends any ? 0 : never] // Hack to recurse.
    : T;

type Result = Unwrap<I>;
// type Result = "e" | "a" | "b" | "c" | "f" | "g";

Playground Link

user_23980200

As listed in the comment above, the Unwrap type works in TypeScript 4.1 (soon-to-be released).

user_24173440
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