Add type operators that can answer "given function type T, what is the type of its return value when arguments P are passed in?"

Search Terms

overload infer function operator arguments parameters return

Suggestion

// Already possible
type ValueForKey<T, K extends keyof T> = T[K];

// Proposal
type Callable = (...args: any[]) => any;
type ValueForArguments<T extends Callable, A extends paramsof T> = T(...A);

Use Cases

This is a proposal that would help with overloaded function issues like #26591 and would help my type testing library be able to support checks like expectTypeOf(fn).toBeCallableWith(args).

Note that though ValueForKey could be implemented as:

type ValueForKey<T, K extends keyof T> = 
  T extends {[Key in K]: infer U} ? U : never;

The following will only work for non-overloaded functions:

type ValueForArguments<T extends Callable, P extends Parameters<T>> = 
  T extends (...args: P) => infer R ? R : never;

since both the inference from Parameters<T> and the type definition will only consider one of the overloads.

What I’m asking is not for inference to consider multiple overloads, which would be nice but might complexify the implementation of type inference, but direct support for paramsof and T(A1, A2, ...A) type syntax which behaves correctly with overloaded functions.

Examples

interface OverloadedFunction {
  (arg: number): number;
  (arg: string): string;
}

// `number`
type Result1 = OverloadedFunction(number);

// `number | string`
type Result2 = OverloadedFunction(number | string);

// `[number] | [string]`
type Result3 = paramsof OverloadedFunction;

// `number | string`
type Result4 = OverloadedFunction(...paramsof OverloadedFunction)

Also

Given the above, it would also make sense to have something like

type Newable = new (...args: any[]) => any;
type ValueForArguments<T extends Newable, A extends paramsof new T> = new T(...A);

where new T takes the constructor type of T and turns it into a function type

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript/JavaScript code
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.

[HitalloExiled]

That was as close as I got to a solution using the current state.

It works in most cases. But has several problems in overloads with optional parameters.

type Calleable = (...args: any[]) => any;
type CallOverloads<T> = T extends
{
    (...args: infer A1):  infer R1,
    (...args: infer A2):  infer R2,
    (...args: infer A3):  infer R3,
    (...args: infer A4):  infer R4,
}
    ? [A1, (...args: A1) => R1] | [A2, (...args: A2) => R2] | [A3, (...args: A3) => R3] | [A4, (...args: A4) => R4]
    : T extends
    {
        (...args: infer A1): infer R1,
        (...args: infer A2): infer R2,
        (...args: infer A3): infer R3,
    }
        ? [A1, (...args: A1) => R1] | [A2, (...args: A2) => R2] | [A3, (...args: A3) => R3]
        : T extends
        {
            (...args: infer A1): infer R1,
            (...args: infer A2): infer R2,
        }
            ? [A1, (...args: A1) => R1] | [A2, (...args: A1) => R2]
            : T extends (...args: infer A1) => infer R1
                ? [A1, (...args: A1) => R1]
                : never;
type Overload<T extends Calleable, TArgs> = Extract<CallOverloads<T>, [TArgs, any]>[1];
type ParameterOverloads<T extends Calleable> = CallOverloads<T>[0];

type TFN1 =
{
    (): void,
    (value: string): number,
    (value: number): boolean,
    (value: number, options: object): object,
}

type T01 = Overload<TFN1, []> // (): void
type T02 = Overload<TFN1, [string]> // (value: string): number
type T03 = Overload<TFN1, [number]> // (value: number): boolean
type T04 = Overload<TFN1, [number, object]> // (value: number, options: object): object
type T05 = ParameterOverloads<TFN1> // [] | [value: string] | [value: number] | [value: number, options: object]

type TFN2 =
{
    (): void,
    (value: boolean): string,
    (value: number, options?: object): object,
}

type T11 = Overload<TFN2, []> // (): void
type T13 = Overload<TFN2, [number, object | undefined]> // never

// Works, but hard to reproduce when the types is provided based on user input
type T12 = Overload<TFN2, [number, object?]> // (value: number, options?: object): object
declare function call<TArgs extends ParameterOverloads<TFN2>>(...args: TArgs): ReturnType<Overload<TFN2, TArgs>>;

call(); // void
call(true) // string
call(1, {}) // never
call(1, undefined) // never

[jcalz]

duplicate of #26043 but that one was closed as a duplicate of #6606 which was ultimately declined; not sure about this one