Can a type be established that references a type parameter from a different line?

Exploring the Promise type with an illustration:

interface Promise<T> {
  then<TResult1 = T, TResult2 = never>(
    onfulfilled?:
      | ((value: T) => TResult1 | PromiseLike<TResult1>)
      | undefined
      | null,
    onrejected?:
      | ((reason: any) => TResult2 | PromiseLike<TResult2>)
      | undefined
      | null
  ): Promise<TResult1 | TResult2>;

  catch<TResult = never>(
    onrejected?:
      | ((reason: any) => TResult | PromiseLike<TResult>)
      | undefined
      | null
  ): Promise<T | TResult>;
}

Could organizing the types for onfulfilled and/or onrejected into separate lines enhance the code's clarity. While not functional in its current state, here is a conceptual representation of what it could resemble:

interface Promise<T> {
  type OnFulfilled = 
      | ((value: T) => TResult1 | PromiseLike<TResult1>)
      | undefined
      | null
  type OnRejected =
      | ((reason: any) => TResult2 | PromiseLike<TResult2>)
      | undefined
      | null

  then<TResult1 = T, TResult2 = never>(
    onfulfilled?: OnFulfilled,
    onrejected?: OnRejected
  ): Promise<TResult1 | TResult2>;

  catch<TResult = never>(
    onrejected?: OnRejected
  ): Promise<T | TResult>;
}

Is there potential to create types similar to this structure to enhance the readability of types?

Answer №1

Currently, there is no way to create "local" or "inline" type aliases using this method. There have been numerous feature requests submitted for this functionality. One request (microsoft/TypeScript#41470) suggests a syntax similar to what you are attempting, while another request (microsoft/TypeScript#23188) proposes a different syntax with the same outcome. Until these features are implemented, you will need to find workarounds.

The closest alternative in TypeScript as of TS5.2 is to create standard non-local, non-inline type aliases and provide them with generic parameters to handle types that are not within scope. For example:

type OnFulfilled<T, R> =
    ((value: T) => R | PromiseLike<R>) | undefined | null;

type OnRejected<R> =
    ((reason: any) => R | PromiseLike<R>) | undefined | null;

interface Promise<T> {
    then<R1 = T, R2 = never>(
        onfulfilled?: OnFulfilled<T, R1>,
        onrejected?: OnRejected<R2>
    ): Promise<R1 | R2>;
    catch<R = never>(
        onrejected?: OnRejected<R>
    ): Promise<T | R>;
}

While this approach introduces new types into the same namespace, it may not be ideal as ambient types such as Promise<T> can clutter the global namespace and potentially conflict with user-defined types. However, depending on the specific use case, it could serve as a feasible solution.

Link to Playground with code

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