fix(runtime-core): watching multiple values - handle undefined as initial values (fix: #5032)

[LinusBorg]

We currently have an empty array as the intial oldValue for a watcher that watches multiple values.

This does fail for an edge case where we watch multiple values and all of them are initially undefined, and trigger the watcher immediately.

We can solve this by settting the initial oldValue to an array filled with INITIAL_WATCHER_VALUE

fix: #5032

[nickrum]

I ran into the exact same issue this PR is fixing. While investigationg the issue, in noticed that, in addition to the original issue, the wrong value is passed to callWithAsyncErrorHandling() when using an immediate watcher with multiple sources over here.

Replacing oldValue === INITIAL_WATCHER_VALUE ? undefined : oldValue with something like (isMultiSource ? (oldValue as []).every((v) => v === INITIAL_WATCHER_VALUE) : oldValue === INITIAL_WATCHER_VALUE) ? undefined : oldValue should fix the issue.

It's a pretty minor issue, so I'm wondering whether this would warrant a separate PR or if you could simply include the fix in your PR.

[antfu]

The test seems not testing the change?

[LinusBorg]

You're right , I'm only testing it's being called initially (which previously it wasn't) but I need to validate the arguments passed.

That actually made me find a part my fix did not cover - oldValue was not undefined when it should be.

[netlify]

❌ Deploy Preview for vuejs-coverage failed.

Name	Link
🔨 Latest commit	1bff8c1
🔍 Latest deploy log	https://app.netlify.com/sites/vuejs-coverage/deploys/633fff8c02245c0008f04572

[zhangbowy]

ant-design-vue fixed this vueComponent/ant-design-vue#6085