change typing of event.this to be never #3147
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developit
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May 15, 2021
MicahZoltu
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Originally, `this` was set to `E['currentTarget']` which, based on the MDN doc linked in preactjs#2166, certainly sounds like the correct thing to do. A couple years later it was changed to `never` in preactjs#3147 with no commentary on the PR or commit indicating why. My guess is that this was changed so arrow functions would work, which I believe are permanently bound and cannot be rebound by the caller. I believe that `never` is wrong because this code doesn't compile: ```ts type TargetedEvent<Target extends EventTarget = EventTarget, TypedEvent extends Event = Event> = Omit<TypedEvent, 'currentTarget'> & { readonly currentTarget: Target } interface EventHandler<E extends TargetedEvent> { (this: never, event: E): void } declare const apple: EventHandler<TargetedEvent<HTMLElement, Event>> declare const event: TargetedEvent<HTMLElement, Event> apple(event) // error TS2684: The 'this' context of type 'void' is not assignable to method's 'this' of type 'never'. ``` Changing to `void` resolves this error and should work everywhere I believe. I removed the comment since it no longer applies with this being `never` or `void`. It appears that it was just forgotten when the switch from `E['currentTarget']` to `never` was made.
JoviDeCroock
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Feb 1, 2023
* Changes `EventHandler<...>` to have a `this` of type `void`. Originally, `this` was set to `E['currentTarget']` which, based on the MDN doc linked in #2166, certainly sounds like the correct thing to do. A couple years later it was changed to `never` in #3147 with no commentary on the PR or commit indicating why. My guess is that this was changed so arrow functions would work, which I believe are permanently bound and cannot be rebound by the caller. I believe that `never` is wrong because this code doesn't compile: ```ts type TargetedEvent<Target extends EventTarget = EventTarget, TypedEvent extends Event = Event> = Omit<TypedEvent, 'currentTarget'> & { readonly currentTarget: Target } interface EventHandler<E extends TargetedEvent> { (this: never, event: E): void } declare const apple: EventHandler<TargetedEvent<HTMLElement, Event>> declare const event: TargetedEvent<HTMLElement, Event> apple(event) // error TS2684: The 'this' context of type 'void' is not assignable to method's 'this' of type 'never'. ``` Changing to `void` resolves this error and should work everywhere I believe. I removed the comment since it no longer applies with this being `never` or `void`. It appears that it was just forgotten when the switch from `E['currentTarget']` to `never` was made. * Removes incorrectly passing test. The type signature's intent is to make it clear to users that they cannot rely on `this` being the element in callbacks, and they should use `event.currentTarget` instead. This test was testing that the user could unwisely ignore that. --------- Co-authored-by: Jovi De Croock <[email protected]>
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