Ability to get generic type from typeof and infer

[ysulyma]

TypeScript version: 3.6.2

Search Terms

typeof generic function return generic type

infer arguments from generic function

Suggestion

I ran into the issue described at #32170, where the answer was

I expect what you're trying to do is refer to a specific call instantiation of a generic function, which isn't currently possible.

The feature request is to make this possible.

Examples

In the following code, dragHelperReact is a helper function for implementing draggable functionality, which also abstracts over Mouse vs Touch events (since Pointer Events are not supported in older browsers). It takes as input three callbacks; since those callbacks have fairly lengthy signatures, I want to infer their argument types. The difficulty is that React.SyntheticEvent is a generic type, which allows narrowing of e.currentTarget.

import * as React from "react";

function dragHelper<T>(
 move: (e: MouseEvent | TouchEvent, coords: {x: number; y: number; dx: number; dy: number}) => void,
 down: (
 e: React.MouseEvent<T> | React.TouchEvent<T>,
 coords: {x: number; y: number},
 upHandler: (e: MouseEvent | TouchEvent) => void,
 moveHandler: (e: MouseEvent | TouchEvent) => void
 ) => void,
 up: (e: MouseEvent | TouchEvent) => void
) {
 return {
 onMouseDown: () => {
 /* implementation */
 }
 };
}

type Args<T extends (...args: any) => any> = T extends (...args: infer A) => ReturnType<T> ? A : void;

type Move = typeof dragHelperReact extends (down: infer M, move: infer D, up: infer U) => void ? M : never;
type Down = typeof dragHelperReact extends (move: infer M, down: infer D, up: infer U) => void ? D : never;
type Up = typeof dragHelperReact extends (move: infer M, down: infer D, up: infer U) => void ? U : never;

// these work
type MoveArgs = Args<Move>;
type UpArgs = Args<Up>;

// I want to do something like this; it does not work.
type DownArgs<T> = Args<Down<T>>;

// This is a workaround I tried; it does not work either.
type DownArgs<T> = Down extends (e: any, ...args: infer U) => void ? [React.MouseEvent<T> | React.TouchEvent<T>, ...U] : never;

// This workaround works.
type DownArgs<T> = Down extends (e: any, ...args: infer U) => void ? [React.MouseEvent<T> | React.TouchEvent<T>, U[0], U[1], U[2]] : never;

class Example extends React.Component {
 down(...[e, coords, upHandler, downHandler]: DownArgs<HTMLDivElement>) {
 // e.currentTarget is narrowed to HTMLDivElement
 }
 move(...[e, coords]: MoveArgs) {}

 up(...[e]: UpArgs) {}

 render() {
 return (
 <div {...dragHelper(this.move, this.down, this.up)}/>
 );
 }
}

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.

[jcalz]

You can kind of get this with some dummy code that appears at runtime, thanks to the recent support for higher order function inference. Note that I'm going to use a non-react-specific example here.

Say you have a generic function like this:

declare function someGenericFunction<T>(
 a0: (cb: string) => number,
 a1: (cb: Array<T>, someNum: number, someVal: T) => number,
 a2: (cb: T) => Array<T>
): string;

and you want some way to represent the argument callbacks' arguments without having the T type parameter resolved to unknown. Well, you can declare this function and class, neither of which will actually be used at runtime:

declare function params<A extends any[]>(f: (...a: A) => any): () => A;

class SomeGeneric<T> {
 args = params(someGenericFunction)<T>();
}

And suddenly the types you want are available:

type A0Params<T> = Parameters<SomeGeneric<T>["args"][0]>; // [string] 
type A1Params<T> = Parameters<SomeGeneric<T>["args"][1]>; // [T[], number, T]
type A2Params<T> = Parameters<SomeGeneric<T>["args"][2]>; // [T]

And you can fill in the T with something other than unknown:

type ConcreteA1Params = A1Params<boolean>; // [boolean[], number, boolean]

The type of params uses higher order function inference to take in a generic function f and spit out another generic function whose return type is the same as f's input parameter tuple. (Note that you don't want to try to use params() at runtime, even though it will be emitted, since it's just declared into existence, and no reasonable implementation of params() could exist.) And the type of SomeGeneric<T> lets you transfer the type parameter from the generic function (which has a concrete type) to a generic type. It's weird, I know.

[suchipi]

Ran into this, too:

function something<T>() {
 const array: T[] = [];
 return array;
}

// Doesn't work; syntax error
type Foo = ReturnType<typeof something<number>>;

I don't know how to best handle specific instantiations of generic functions on their own (eg typeof something<number>, but maybe ReturnType and Parameters could accept optional additional type parameters for this?

// Proposal (not working today)
type Foo = ReturnType<typeof something, number>;

[Xunnamius]

Also ran into this trying to get the parameters of a default import function.

[thany]

Just ran into this problem the other day.

How could you possibly have typeof not return a generic type for a generic function? This seems like a pretty severe bug to me, as it introduces type inconsistency, technically speaking.

[ahejlsberg]

With #47607 it is possible to refer to specific instantiations of generic functions.

[mikeaustin]

Somewhat related, and I did extensive Googling, I just can't find how to apply a generic parameter default type when using typeof. Am I missing something basic?

type ViewProps<T extends React.ElementType = 'div'> = {
 as?: T,
} & React.ComponentPropsWithoutRef<T>;

const View = <T extends React.ElementType = 'div'>(...): ViewProps<T>) => { ... }

React.ComponentProps<typeof View<'div'>>; // This works since passing type parameter explicitly

Without passing the generic parameter type 'div', you get the generic function, which results in Omit<any, 'ref'> an no props being checked at all.

React.ComponentProps<typeof View>; // I'm not sure how to apply/find the generic parameter default here