Difficulties with Typescript args type inference

Question

I am having trouble understanding an inference error. The ts linter underline (event: E) of useCallback with the message bellow. When I cast the callback of useCallback with as T the linter message is gone. Is there any way to avoid doing this? What did I didn't understand?

Complete code:

import { DependencyList, useCallback } from 'react'

const useStopBubblingCallback = <T extends (event: E) => any, E extends Event = Event>(
  callback: T,
  deps: DependencyList
) : T =>
  useCallback<T>((event: E) => {
    event.stopPropagation()
    event.preventDefault()
    return callback(event)
  }, deps)

export default useStopBubblingCallback

Linter message:

Argument of type '(event: E) => any' is not assignable to parameter of type 'T'.
  '(event: E) => any' is assignable to the constraint of type 'T', 
but 'T' could be instantiated with a different subtype of constraint 
'(event: E) => any'.ts(2345)

bela53 · Accepted Answer · 2020-10-02 09:51:03Z

2

You can change useStopBubblingCallback to the following signature (example):

const useStopBubblingCallback = <E extends Event = Event>(
  callback: (event: E) => any,
  deps: DependencyList
) : (event: E) => any =>
  useCallback((event: E) => { /*your code*/ }, deps)

const cb = useStopBubblingCallback((event: MouseEvent) => {}, []) 
// cb: (event: MouseEvent) => any

Note, that in your case type parameter E had no inference candidate in the outer function, so TypeScript cannot do anything useful with E (it would be fixed to Event all the time).

We also drop the manual type argument in useCallback<T> to let the compiler automatically infer its type.

answered Oct 2, 2020 at 9:51

bela53

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