prepend method infers payload type from the function argument if it exists... and if there is no argument the resulting payload type will be of type unknown. This is a bug because a user cannot call Event<unknown> without params - it waits for 1 argument with type unknown.

const event = createEvent<string>();
const prepended = event.prepend(() => 'foo') // Event<unknown>
prepended() // TS error: Expected 1 arguments, but got 0.

To bypass the type error, a user needs to specify void manually:

const event = createEvent<string>();
const prepended = event.prepend<void>(() => 'foo') // Event<void>
prepended() // ok

But we can fix this by adding a default type to prepend's Before:

  prepend<Before = void>(fn: (_: Before) => Payload): Event<Before>

As the result, TypeScript will show an error when you directly call Event<void> with an argument, which is good too:

const event = createEvent<string>()
const prepended = event.prepend(() => 'foo')
prepended(1) // Argument of type 'number' is not assignable to parameter of type 'void'.

Now it can be seen that the addition of the default value improves typing in both cases.

P.S. I also added type tests for .prepend and the described cases.