Typescript: Get the type of the last parameter of a function type

Question

Assume I have a function type, e.g.

type somefntype = (a: number, b: string, c: boolean) => void

I need the type of that function types

Answer 1

Here is yet a another solution which is a slight modification of jcalz answer to make it more generic using some recursive conditional types:

type Head = T extends [any, ...any[]] ? T[0] : never;
type Tail =
    ((...t: T) => any) extends ((_: any, ...tail: infer U) => any)
        ? U
        : [];
type HasTail = T extends ([] | [any]) ? false : true;

type Last = {
    0: Last>
    1: Head
}[
    HasTail extends true ? 0 : 1
];

type LastType = Last>; // boolean

It might be interesting to consider replacing any with unknown as well.