Specify Order of Variadic Generic Arguments for Variable Parameters in TypeScript

Question

I'm trying to do something that feels absurd, but I've gotten so close I feel it must be possible, I just can't quite get it.

I'm trying to create a generic function type such that assigning the function type

const typedFunction: Generic<
    SomeObject,
    ['keys', 'in', 'that', 'object']
>;

will produce a function whose arguments are typed, exactly:

(
    arg0: typeof SomeObject['keys'],
    arg1: typeof SomeObject['in'],
    arg2: typeof SomeObject['that'],
    arg3: typeof SomeObject['object']
)

I've arrived at two near-but-not-quite solutions;

1.

declare type SomeGeneric<
   T,
   Params extends (T[keyof T])[]
> = (...args: Params) => ReturnType;

Which preserves the order, but forces me to specify Params in a very ugly way:

const someFunction: SomeGeneric<
    SomeObject,
    [SomeObject['keys'], SomeObject['in'], ...]
> = (arg0, arg1, arg2, arg3) => { ... };

This is obviously verbose to the point of being useless, as I could just specify them directly that way.

2.

declare type SomeGeneric<
    T,
    Params extends (keyof T)[]
> = (...args: T[Params[number]][]) => ReturnType;

But, since Params[number] refers to any member of Params, this turns every argument of SomeGeneric<SomeObject, ['keys', 'in', 'that', 'object']> into a union type of SomeObject['keys'] | SomeObject['in'] | ...

What I'd like to know is if there's some way to specify that I want SomeObject to be accessed via the keys provided in order, something like ...args: SomeObject[...Params[number]] if such syntax weren't nonsensical.

Tobias S. · Accepted Answer · 2022-11-22 09:59:42Z

1

You were pretty close with your 2nd attempt.

You need to use mapped type to map over the elements in Params. For each index I you can get the array element Params[I] which can be used to T.

Note that using a mapped type to map over tuple also produces a tuple since 3.1. This is essential here since a spread parameter type must be a tuple type.

declare type SomeGeneric<
   T,
   Params extends (keyof T)[]
> = (...args: { [I in keyof Params]: T[Params[I]] }) => void;


type Test = {
    a: string
    b: number
    c: Date
}

const someFunction: SomeGeneric<Test, ["a", "b", "c"]> = (
    arg0, // string
    arg1, // number
    arg2  // Date
) => {}

Playground

edited Nov 22, 2022 at 9:59

answered Nov 22, 2022 at 9:53

Tobias S.

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3 Comments

JMA Over a year ago

Aha, I was trying to use X in keyof Y but I had no idea you could specify variable arguments as an object type rather than an array type. Thank you!

Tobias S. Over a year ago

@JMA - it is a tuple type. See the 3rd paragraph (that I just edited in)

JMA Over a year ago

that makes more sense; the fixed/const nature of tuples is exactly why this works, and I understand that now.

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