Typescript state auto-typing and implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer

[vmihailenco]

When using TypeScript and vue-cli the following classic example does not compile with TypeScript 3.6.3:

export function useCounter() {
  const state = reactive({
    count: 0,
    double: computed(() => state.count * 2),
  })
  return state
}

because 'state' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.. I can "fix" it with following code but I wonder if there are better ways without resorting to manually creating a type definition for the state:

export function useCounter() {
  let self: any = null
  const state = reactive({
    count: 0,
    double: computed(() => self.count * 2),
  })
  self = state
  return state
}

After all that is a common case and it does not work as expected...

[dvic]

I'm encountering the same problem, currently I do something like

interface Counter {
  count: number,
  double: number
}

export function useCounter() {
  const state = reactive({
    count: 0,
    double: computed(() => state.count * 2)
  }) as Counter;
  return state
}

[vmihailenco]

Yeah, the advantage of that approach is that we can use Counter interface elsewhere when passing state to another function. The problem is obvious - we need to maintain code in 2 places.

Supporting for classes would be neat e.g.:

class Counter {
  count = 0
  double = computed(() => this.count * 2)

  increment() {
    this.count++
  }
}

export function useCounter() {
  const state = reactive(new Counter())
  return state
}

[drewstewart]

what do you mean you need to maintain code in 2 places? defining types / interfaces describing shape of your data seems completely acceptable.

[liximomo]

There seems to be no clean way to fix, as TS really can't handle the type of something self referring.

[lbssousa]

I think it's better to use ref in this case:

export function useCounter() {
  const count = ref(0)
  const double = computed(() => count.value * 2)

  return {
    count,
    double
  }
}

[liximomo]

Good news, TS 3.7 will support Recursive Type References.

[liximomo]

@vmihailenco What do you mean that "class is not supported"?

[vmihailenco]

I mean that following does not work for me:

class Counter {
  count = 0
  double = computed(() => this.count * 2)

  increment() {
    this.count++
  }
}

export function useCounter() {
  const state = reactive(new Counter())
  return state
}

function useSomeHook(state: Counter) {} // <- solved

When calling increment this seems to be null/undefined. I did not try, but computed is likely broken too. I tried naive fix like state.increment.bind(state) but it did not work out.