Helper to assert that an event is the expected type

[VanTanev]

Edit: This is now available as part of https://github.com/VanTanev/xstate-helpers

When we have events on actions, this provides a way to assert and type-narrow the event from all possible events on the machine:

import { assign, createMachine, ExtractEvent } from 'xstate'

type Context = {
  date: Date
}

type Event = 
 | { type: 'SHOW_DATE' }
 | { type: 'UPDATE_DATE', date: Date }

const machine = createMachine<Context, Event>({
  on: {
    SHOW_DATE: { actions: 'showDate' },
    UPDATE_DATE: { actions: 'updateDate' }
  }
}, {
  actions: {
    showDate: (ctx, e) => {
      assertEvent(e, 'SHOW_DATE')
      alert(ctx.date.toISOString())
    },
    updateDate: assign({
      date: (_, e) => {
        assertEvent(e, 'UPDATE_DATE')
        return e.date // has type "Date"
      }
    })
  }
})

function assertEvent<
  TEventType extends TEvent['type'], 
  TEvent extends EventObject = EventObject
>(
    e: TEvent,
    type: TEventType,
): asserts e is ExtractEvent<TEvent, TEventType> {
    if (e.type !== type) {
      throw new Error(`Invariant voilation: Expected event "${type}" but instead got "${e.type}"`)
    }
}