Ability to get generic type from typeof and infer

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.

[ahejlsberg]

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

[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>;