Typing return type for react's `useMemo()`

[acoustic timber]

OnkelTem#4669

Playground Link

[sudden adder]

!ts

[acoustic timber]

import { useMemo } from "react";
//                      ^^^^^^^
// Cannot find module 'react' or its corresponding type declarations.

type Foo = { a: string };

useMemo<Foo>(() => {
  return { a: "", b: "" };
}, []);

useMemo((): Foo => {
  return { a: "", b: "" };
//                ^^^^^
// Type '{ a: string; b: string; }' is not assignable to type 'Foo'.
//   Object literal may only specify known properties, and 'b' does not exist in type 'Foo'.
}, []);

[sudden adder]

Erm. not sure about the messages

I don't have them

I wonder why the first variant allows for b

Ideas?

It has a type as:

function useMemo<T>(factory: () => T, deps: DependencyList | undefined): T;

Oh... wait

I stupid

Nevermind

It is generic! Damn