Contextual narrowing not narrowing on generic map.

[whitneyit]

Bug Report

🔎 Search Terms

• contextual narrowing
• type narrowing 4.3
• 4.3

🕗 Version & Regression Information

• This is something that I expected to work after 4.3's Contextual Narrowing for Generics

⏯ Playground Link

Playground link with relevant code

💻 Code

declare const eventType: keyof DocumentEventMap;

document.addEventListener(eventType, (event) => {
 event; // Here `event` is `Event | UIEvent | AnimationEvent | MouseEvent | InputEvent | FocusEvent | CompositionEvent | ... 9 more ... | ClipboardEvent
 if (eventType === `click`) {
 event; // Expected: `event` to be type `MouseEvent`. Actual: It's the union from above.
 }
});

🙁 Actual behavior

Second reference of event is type Event | UIEvent | AnimationEvent | MouseEvent | InputEvent | FocusEvent | CompositionEvent | ... 9 more ... | ClipboardEvent.

🙂 Expected behavior

Second reference of event to be of type MouseEvent.

[jcalz]

Generic functions like document.addEventListener() have their type parameters specified when you call them. In the case above, the K type parameter is inferred to be keyof DocumentEventMap and therefore the type of event in the callback is the full union; it is no longer "generic" in the way that #43183 (the contextual narrowing feature) would have any bearing on.

It seems like what you really want is something like #27808 where the type parameter K is treated as some unknown single member of the keyof DocumentEventMap union... although then the compiler might not even let you call document.addEventListener() with a union-typed eventType.

This also feels like eventType and event are supposed to be correlated unions but the compiler has no way to track that; which is #30581. (This is where #25051 would have helped, if you could wrap the call to document.addEventListener() in a hypothetical type switch (eventType){...} block. Oh well!)

[whitneyit]

Thanks @jcalz for the detailed response! I agree that this is similar to #30581.

Unlike #30581, I was under the impression that the if check would be sufficient to narrow the type to MouseEvent.

[fatcerberus]

Right - the compiler doesn’t understand that eventType and event are interrelated for the purposes of type narrowing.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.