TypeScript function argument generics

Question

I'm trying to create a TypeScript function that accepts an array and returns a new function where the arguments have the types of the array.

In other words, it should be something like this:

type MyFunc = {
    <A>(arg: [A]): ((arg0: A) => any)
    <A, B>(arg: [A, B]): ((arg0: A, arg1: B) => any)
    <A, B, C>(arg: [A, B, C]): ((arg0: A, arg1: B, arg2: C) => any)
}

But for a variable number of inputs

Concrete examples

Here's some concrete examples (even though I'm looking for the general case):

type MyFuncA = (arg: [number]) => ((arg0: number) => any)
type MyFuncB = (arg: [string]) => ((arg0: string) => any)
type MyFuncC = (arg: [number, string]) => ((arg0: number, arg1: string) => any)
type MyFuncD = (arg: [number, string, bool]) => ((arg0: number, arg1: string, arg2: bool) => any)

Alex Wayne · Accepted Answer · 2022-04-17 01:43:18Z

I think the simplest would be:

type MyFunc = <T extends readonly unknown[]>(args: T) => (...args: T) => unknown

It accepts an array of anything as T, and spreads that array over the arguments of the returned function with ...args: T.

Usage:

declare const fn: MyFunc // pretend this function exists

const testA = fn([1, 'asd', true] as const)
// (args_0: 1, args_1: "asd", args_2: true) => unknown

Just note the as const is used here to force the argument to be a tuple type [number, string, boolean], rather than an array type of (number | string | boolean)[], since it needs the tuple to infer the order of the arguments. But providing any tuple type should work fine.

If you don't pass in a tuple, you will provide a spread array type which is better than nothing but not as useful:

const testA = fn([1, 'asd', true])
// (...args: (string | number | boolean)[]) => unknown

Playground

If you have a tuple type with names for each element, those names will even be reported as the argument names of the function!

const myTuple: [num: number, str: string, isAwesome: boolean] = [1, 'asd', true]
const testA = fn(myTuple)
// (num: number, str: string, isAwesome: boolean) => unknown

Which is neat.

Playground

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