Flip Arguments

In this medium-level challenge, you'll implement FlipArguments<T> which takes a function type and returns a new function type with the same return type but with its parameters reversed.

type Flipped = FlipArguments<(arg0: string, arg1: number, arg2: boolean) => void>
// (arg0: boolean, arg1: number, arg2: string) => void

Detailed Explanation

We need a helper to reverse a tuple, then apply it to the function's parameter list.

type Reverse<T extends any[]> = T extends [infer First, ...infer Rest]
  ? [...Reverse<Rest>, First]
  : []
 
type FlipArguments<T extends (...args: any[]) => any> =
  T extends (...args: infer Args) => infer R
    ? (...args: Reverse<Args>) => R
    : never

How it works:

• Reverse<T> recursively reverses a tuple by taking the first element, reversing the rest, and appending the first element at the end
• T extends (...args: infer Args) => infer R extracts the parameter tuple Args and return type R from the function type
• We construct a new function type using Reverse<Args> as the parameters and R as the return type
• The constraint T extends (...args: any[]) => any ensures that only function types are accepted, causing non-function inputs to produce a type error

An alternative approach uses a single type without a separate Reverse helper:

type FlipArguments<T extends (...args: any[]) => any> =
  T extends (...args: infer Args) => infer R
    ? (...args: Args extends [...infer Rest, infer Last]
        ? [Last, ...FlipArguments<(...a: Rest) => R> extends (...a: infer P) => any ? P : never]
        : []) => R
    : never

However, the two-type approach with a separate Reverse is cleaner and easier to understand.

This challenge helps you understand function type manipulation with parameter tuple reversal and how to apply this concept in real-world scenarios.