Why is `BoundEventHandler` typed this way?

The type for BoundEventHander is,

  interface BoundEventHandler<T, E extends Event> {
    0: (
      data: any,
      e: E & {
        currentTarget: T;
        target: DOMElement;
      }
    ) => void;
    1: any;
  }

  interface BoundEventHandler<T, E extends Event> {
    0: (
      data: any,
      e: E & {
        currentTarget: T;
        target: DOMElement;
      }
    ) => void;
    1: any;
  }

In order to use it, I need to do something like const handler = BoundEventHandler<T,E>[0], note the [0] at the end, which seems weird. Why is it typed this way? How is this type expected to be used?

5 Replies

thetarnav•3mo ago

this is for the onClick={[callback, data]} syntax

aryzing•3mo ago

So it's normal to use [0] when typing a bound event handler function? Since the interface has Handler in the name, it would seem that it is the type for a function, not a tuple

lxsmnsyc•3mo ago

it is a tuple type, but it's declared this way since interfaces that are redeclared extend their own types whether or not it needs to be typed this way can be up for discussion though

aryzing•3mo ago

Thanks @lxsmnsyc 🤖 . I guess I asked about the type because I was a bit surprised by the type syntax needed to type the handler, although perhaps a better question is how are handlers typically typed?

lxsmnsyc•3mo ago

it's either just the plain callback or that it's a union, so to speak

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Why is `BoundEventHandler` typed this way?