[bug]: EventHandler type should allow boolean types

[richardaum]

Which react-spring target are you using?

• @react-spring/web
• @react-spring/three
• @react-spring/native
• @react-spring/konva
• @react-spring/zdog

What version of react-spring are you using?

9.7.4

What's Wrong?

If we use any callback lets say for useTransition hook, type inference will fail if you use boolean as data. While investigating the root cause, I stumble upon this EventHandler. By reducing it to a single parameter function definition I could identify the issue with boolean type.

type EventHandler<Item = undefined> = 
  Item extends undefined ? 
    (item?: Item) => void : 
    (item: Item) => void;

To Reproduce

If you try to assign EventHandler to an arrow function to see the error happening, you'll notice the issue, as pointed in here, but also below:

type EventHandler<Item = undefined> = Item extends undefined ? (item?: Item) => void : (item: Item) => void;

const failingEventHandlerInference: EventHandler<boolean> = (item) => { } // fails
const goodEventHandlerInference: EventHandler<string> = (item) => { } // succeeds

Expected Behaviour

The real issue is that Item extends undefined that simply doesn't work with boolean type. Instead, as I expected both source code and who uses EventHandlers doesn't simply omit the item argument but provides a undefined-ish value, we could do:

type EventHandler<Item = undefined> = 
  (item: Item) => void

That will still allow Item to be inferred properly. After locally testing it by applying that change directly on my node_modules/@react-spring/types/dist/react-spring_types.modern.d.ts, it worked. As I simplified the EventHandler the actual change would be:

type EventHandler<TResult extends Readable = any, TSource = unknown, Item = undefined> = (result: AnimationResult<TResult>, ctrl: TSource, item: Item | undefined) => void;

Source code

Link to repo

• https://codesandbox.io/p/sandbox/react-typescript-forked-xf43l4
• https://www.typescriptlang.org/play/?#code/C4TwDgpgBAogbhAdsAEgQ0QEwDYQE4A8AksBALZQC8UArlhAGYCWiEmAfFVCeVBAB6ksAZ1r1mrTFAD8UABRNSZaQC5uSgJRVOcAPZMpahUrU8yWyjv2YA3ACg7AY12JhwKAzRNsLAObwkVAwcfCJEBnwkRwg1AOR0LFxCACNdXVwMTmpjcgtOAG8oAF8oAHpSjy9sYScXNyhfNMw4oMTQ8MjEaNiEeOCkgjc8Pyz5RVztKEKS8qhhGkdotmEgA