What is the method for extracting the types of parameters from a function type while excluding a single parameter?

Suppose I have the following interface that defines a function:

export declare interface NavigationGuard {
    (to: RouteLocationNormalized, from: RouteLocationNormalized, next: NavigationGuardNext): NavigationGuardReturn | Promise<NavigationGuardReturn>;
}

The goal is to create this type:

export SomeNavigationGuardType = (to: RouteLocationNormalized, from: RouteLocationNormalized): NavigationGuardReturn | Promise<NavigationGuardReturn>

Working with the original NavigationGuard type ...

I've attempted to use the utility type Parameters, extracting and utilizing Omit to remove the next parameter, but the Parameters utility type extracts to a tuple.

I've tried using:

type NavigationGuardParams = Omit<Parameters<NavigationGuard>, 'next'>

export type UseRouterNivigationGuards = {
  beforeEach?: (arg: NavigationGuardParams) => ReturnType<NavigationGuard>
  beforeResolve?: (arg: NavigationGuardParams) => ReturnType<NavigationGuard>
  afterEach?: NavigationHookAfter
}

and

type NavigationGuardParams = Omit<Parameters<NavigationGuard>, 'next'>

export type UseRouterNivigationGuards = {
  beforeEach?: (...arg: NavigationGuardParams) => ReturnType<NavigationGuard>
  beforeResolve?: (...arg: NavigationGuardParams) => ReturnType<NavigationGuard>
  afterEach?: NavigationHookAfter
}

However, both attempts result in a usage/parameter error, with the first giving this message:

Expected 1 argument, but received 2.

The second attempt produces:

Argument of type '[RouteLocationNormalized, RouteLocationNormalized]' cannot be assigned to a parameter of type 'NavigationGuardParams'.

Is there a workaround to achieve this?

Answer №1

To eliminate the next component from the tuple that is produced by the function Parameters, you can implement a conditional type combined with infer:

type ExcludeNextElement = (...args: Parameters<NavigationGuard> extends [...infer T, next: unknown] ? T : never) => ReturnType<NavigationGuard>

Answer №2

To easily access the elements in the Parameters array, you can reference them by their index:

const SomeGuardFunction = (to: Parameters<NavigationGuard>[0], from: Parameters<NavigationGuard>[1]) => ReturnType<NavigationGuard>

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