xstate helpers

assertEvent.ts

export function assertEventType<
  TE extends EventObject,
  TType extends TE['type']
>(event: TE, eventType: TType): asserts event is TE & { type: TType } {
  if (event.type !== eventType) {
    throw new Error(
      `Invalid event: expected "${eventType}", got "${event.type}"`,
    )
  }
}

assignWith.ts

import {
  assign,
  Assigner,
  EventObject,
  PropertyAssigner,
  PureAction,
} from 'xstate'

export const assignWhen = <
  TContext,
  TEvent extends EventObject,
  TEventType extends TEvent['type']
>(
  type: TEventType,
  assignment:
    | Assigner<TContext, Extract<TEvent, { type: TEventType }>>
    | PropertyAssigner<TContext, Extract<TEvent, { type: TEventType }>>,
): PureAction<TContext, TEvent> =>
  // @ts-expect-error not sure about this
  actions.pure((context, event) => {
    if (event.type !== type)
      throw new TypeError(`Event type does not match "${type}"`)
    return assign(assignment)
  })