Append Argument

Disclosure

Most of the problems under my TypeHero Challenges folder were either obtained from typehero.dev or from type-challenges repo. Purpose of these articles are just to document my approaches for my easy reference. Please visit the respective links for more info.

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Problem Description

For given function type Fn, and any type A (any in this context means we don’t restrict the type, and I don’t have in mind any type 😉) create a generic type which will take Fn as the first argument, A as the second, and will produce function type G which will be the same as Fn but with appended argument A as a last one.

For example:

type Fn = (a: number, b: string) => number
 
type Result = AppendArgument<Fn, boolean> 
// expected be (a: number, b: string, x: boolean) => number

Solutions

Approach 1

type AppendArgument<Fn extends Function, A> = Fn extends (...args: infer Args) => infer R
	? (...args: [...Args, A]) => R
	: never;