How to narrow down context type by state in xstate v5? (typestates or something else?)

[nickredmark]

In v4 it was possible to narrow down context type by state - how does one do this in v5?
https://xstate.js.org/docs/guides/typescript.html#typestates

[crishoj]

Being able to narrow down the context type by state would be useful.

As a workaround for actor input from context, I am using a small fromContext helper:

{ 
 Moving: {
    invoke: {
      src: 'moveItem',
      input: fromContext('item', 'strategy'),
      onDone: { target: 'Run', actions: 'clearItem' },
      onError: 'Recovering',
    },
  },
}

fromContext returns a struct with a type assertion that the specified keys are present:

export function fromContext<T extends object, K extends keyof T>(...keys: K[]) {
  return ({ context }: { context: T }) => assertPick(context, keys)
}

export function assertPick<T extends object, K extends keyof T>(obj: T, keys: K[]): Required<Pick<T, K>> {
  const result = pick(obj, keys) as T & Required<Pick<T, K>>
  return assertKeys(result, keys)
}

export function assertKeys<T extends object, K extends keyof T>(obj: T, keys: K[]): T & Required<Pick<T, K>> {
  for (const key of keys) {
    if (obj[key] === undefined)
      throw new Error(`Missing key '${String(key)}', expected [${keys}], found [${Object.keys(shake(obj))}]`)
  }
  return obj as T & Required<Pick<T, K>>
}