throw `Cannot read properties of undefined (reading 'stop')` on component unmount, and use nested watch.

[BiosSun]

Version

3.2.30

Reproduction link

sfc.vuejs.org/

Steps to reproduce

1. please click 'Toggle' button in demo, you will see the error.

What is expected?

When the 'Toggle' button is clicked, the <Comp> component is unmounted normally.

What is actually happening?

'Toggle' button then change visible to false, then it will cause the <Comp> to be unmount, and throw error.

---

The issue is present in 3.2.30 and later.

[posva]

I don't think you are supposed to call the watch within a watch because then they are not related. You could however move the watch outside or create a nested effectScope to create watchers whenever you want like this

[BiosSun]

Yes, there are many other ways.

But in my project, I want to do is when a property (e.g. visible) of the component is toggled to true, turn on some function, and start watching for the change of state related to this function. And when it toggled to false , the function is turn off, and stop watch.

So, "naturally" I use nested watch. (There may be other, more natural and better ways, which I don't know of.)

Also, since this is an unreasonable way and it does cause an error, is there any way to stop people using it?

[posva]

You can use the solution I showed you at the moment.

I don't know if it makes sense to activate the watcher effectScope within a running watcher

[edison1105]

@posva @LinusBorg
I feel this is a bug,because effects may be changed when an effect stoped.thats why the error is shown.
2993a24#diff-ac9e1628a205e241a9796fcffbdb27ae0dda80494c45a0830ee67309a1e3052fR57-R66

[Zelig880]

Hi guys, I just wanted to know that I was going to raise this precise bug.

During Unit Test development we have started to encounter this issue after upgrading from 3.2.27 to 3.2.37. The change that breaks the test is the one highlighted above.

In the previous implementation this.effects.forEach(e => e.stop()) would run with no problem, but in the current implementation the value of "l" changes and breaks the loop.

After some investigation, we have noticed that the value of "l" (the variable used to handle the length of the array) and the actual array length, do not match and this is resulting in undefined (I have not analysed the reason behind this).

The following screenshot shows the console log and its miss-alignement.

Updating the for loop without reusing the "l" variable would fix the issue, but I know that the problem is somewhere else and this is not the solution, but i wanted to provide you all the context that I could as I cannot create a reproducible code.

for (i = 0, i = this.effects.length; i++) {
    this.effects[i].stop();
}