Type of watch() doesn't take first run into account

[jacekkarczmarczyk]

Version

3.0.0-alpha.4

Reproduction link

https://codepen.io/jkarczm/pen/ZEYgEXd?editors=0011

Steps to reproduce

import { ref, watch } from "vue";

const r = ref('foo')

watch(r, (_, oldVal) => console.log(oldVal))

watch callback is defined as

(value: T, oldValue: T, onCleanup: CleanupRegistrator) => any

https://github.com/vuejs/vue-next/blob/master/packages/runtime-core/src/apiWatch.ts#L40-L44

It means that in this case it will be

(value: string, oldValue: string, onCleanup: CleanupRegistrator) => any

However on the first run oldVal is undefined.

What is expected?

When I want to use oldVal, for example console.log(oldVal.length) I should be warned by TS that the value can be undefined

What is actually happening?

No warning

[ShenQingchuan]

Do you mean that this is supposed to be like this ?

export type WatchCallback<T = any> = (
  value: T,
  oldValue: T | undefined,
  onCleanup: CleanupRegistrator
) => any

[jacekkarczmarczyk]

The problem with this solution is that oldValue can be unnecessarily undefined for consecutive calls, but to be honest I have no idea if there is a different way to solve it.

[KaelWD]

Presumably you'd be checking that anyway with a type guard so undefined would be removed.

Also if options.lazy === true then oldValue should be T